Defragmenting the hard drive if it is very fragmented can often speed up your computer. So can fixing registry problems. You can disable features and services you are not using. For instance, if you are not big into visual effects, you can disable most of them to get more performance. If you are not using a laptop or don't have "green" motivations, then you can turn off a lot of the power management stuff to increase performance.
You can also make some hardware changes too. If you can install faster memory or a faster CPU that can help. If you are running low on memory a lot, then you can add more memory so the hard drive won't have to be used for temp files a lot. Then you may want to install a faster hard drive such as a SSD drive at least for use as a system drive. You can also try adding a USB memory stick under Windows 7/8 to use for ReadyBoost.
A computer can be made faster by:
1. Defragging it regularly using Windows Disk Defragmenter.
2. Deleting the temporary files in the system.
3. Reducing the number of programs that start during windows start up.
4. Optimizing system configuration.
5. Adding more RAM (memory).
6. Upgrading the CPU.
7. Upgrading other components such as the graphic card.
Without spending money:
Eliminate all the extraneous software from your machine. Nearly everything you download these days wants to run an "agent" on startup. All it does is sit there and soak up RAM and CPU cycles. If your system tray (down by the clock) is crowded, get rid of as much of that stuff as you can. Additionally there are many gray-area malware apps (spyware adware scareware) which may or may not be picked up by your virus scanner
Below are links to a couple good programs for removing this stuff. First is "Malware Bytes," it will help you find any malicious or semi-malicious programs running on your machine and remove them for you. Second is "Decrapifier." It is intended to be used on a newly purchased computer and will help remove all the pre-packaged demo-ware which comes with new computers.
Be sure you have at least 10% free space on your hard disk and then defrag your hard disk. The defrag utility is in start > accessories > system tools. Or go to My Computer, right click on the drive letter in question, and go to properties. There you should find the defragger and the disk checker.
If you don't mind spending some money:
Adding system RAM can greatly improve the performance of any computer system. Its like putting a turbo-charger on your car, the engine is the same, but it produces a lot more power. You can never have enough RAM for Windows, and adding RAM is always a good way to get a little more performance out of your machine.
Upgrading your video card is also a good way to boost performance in 3d games. Modern video cards have come a long way in the last couple years and a decent 512mb card can be had for under $100.
Improving machines that once performed well:
If your machine used to run better, but is running slow now, your best bet is to do a few system-maintenence measures to speed things up a bit.
First, do a good spyware scan. Spybot Search & Destroy or Lavasoft Adaware are both excellent freeware programs which will scan your system for useless software and remove it automatically. Both are linked below.
Second, make sure you don't have infections. There are plenty of antivirus tools for this.
Third, see all that stuff down in your system tray (on the start bar by the clock), all that stuff is qeued up to run on startup and then just sit there soaking up RAM and CPU cycles. Go through those programs and through their prefs and tell them to stop loading on startup. Failing that, Spybot S&D also has an excellent utility for removing unwanted programs from the startup routine.
Fourth, optimize your operating system. Windows by default loads up all sorts of services that may or may not be necessary for your particular usage needs. I suggest searching for "Windows XX optimization guide" with XX as your Windows version for more info on what you do and dont need.
Fifth, and maybe this should be first. Unplug your machine, remove the cover, and blow out all those dust-bunnies. A clean computer is a cool computer, and a cool computer is free to operate faster.
How hard drives affect performance
Getting the most performance from your PC requires that all its components are operating at their full capacity at all times.
By far, the slowest component in any system is the hard drive, since it is mechanical and is in constant use (even if when only web browsing many pages download temp files to your system to access their content, causing items to be written and read).
Perhaps the biggest destroyer of hard drive performance is fragmentation, as having pieces of files and free space randomly scattered all over your disk necessitates a lot more disk action to write and later access the files than would otherwise be needed (even insufficient RAM boils down to slowed disk performance as the page file will have to be used more when RAM is low). Keep the HDD well organized. The HDD can be the culprit on an otherwise well equipped system. Be careful about downloading and installing the programs and defragment regularly to keep data access smooth. In fact you can even automate the task or get a defrag program that will proactively tackle fragmentation. Besides this, run a disk cleanup often and ensure your security updates are in place.
So while getting more RAM will certainly help (mainly because the disk will be used less until the RAM is used up), the real bottleneck occurs at the hard drive -- it's the weakest link in any system.
That being said, the best investment would go in the direction of getting and using advanced third party defrag solutions that can automatically defrag your disks as needed, not over night or "once a month", but now, when the disk is being used.
Here is a link to a Top 10 Review which compared the leading defrag programs and ranked them according to performance and value (their Gold Medal winner, Diskeeper, is the only solution available which actually prevents fragmentation). You can get a trial for the gold medalist.
These appear in the links below under "Top 10 Review of Defrag Solutions" and "Defrag".