Yes, you can.
Install Win XP on the first hard drive, then install Vista from Xp and on question about "what kind of installation do you want to have?" Answer "separate" installation. When it asks you "which hard drive are you going to use?" Choose the second one. Vista will create a boot list automatically.
It is best to wipe the hard drive before you remove it from the first computer and do a new install for the new computer. Then add your operation system.
You can install XP at 98 direct. Means there is no requirement to format 98. Go into your computer's BIOS and set the master boot device as the CD ROM drive. Insert the winxp disk and restart. From the screen that appears you should be able to format the Hard drive, convert the file system from FAT32 into NTFS, and install windows XP.
There are several ways to do this. Perhaps the easiest is to buy an external housing for the drive, and connect it via USB to the WinXP Pro computer. There are many such USB hard drive housings. Look for 'external USB hard drive enclosures' or 'external USB hard drive cases'. This is easy to do; it assumes that the WinXP machine has a USB port, a very reasonable assumption these days (2005). It also provides flexibility, as you can then use that hard drive from the Win 98 machine on any computer with a USB port. The traditional way to do it is to open up the WinXP Pro machine's box and see if there is room to physically locate the drive in the machine, and also to see if there is a connector type for the hard drive you want to install (from the Win98 machine), most likely a 40-pin IDE connector. Then its a matter of setting jumpers on the drive, usually to change it from 'master' to 'slave' mode, and connecting the ribbon cable between the computer's motherboard and the disk drive.
For the users of Win 7 Ultimate or Enterprise or Win 8, they can use BitLocker to lock hard drive. But for people on other OSes, they can only lock the hard drive with 3rd party software such as Kakasoft Advanced Folder Encryption.
you have to install win xp via a usb or by using an external CD drive.
Drive 'c' is the hard drive that the operating systems (Windows XP/ Vista/ Win 7 etc) is stored.
The most mobile hard drive case available on the market is the Win Ammo Hard Drive Enclosure. This hard drive casing is thick and will protect your hard drive well as you transport it. Additionally, this hard drive comes with a built in encryption feature. This feature keeps your hard drive locked until an RFID key is used to unlock it. So if you accidentally lose your hard drive and case, anyone who finds it will find it difficult to access the data on your hard drive.
Clarifying the question. Currently running WinXPPro in this computer, which is not mine, I want to connect my HHD (350gig) temporaly and be able to choose from which to boot. My HHD as XP Home, will it affect my lincense from Microsoft?
It sounds like the drive is too large for Win 98 to recognize. I've seen varying accounts of the largest drive Win 98 will handle, but the most common seems to be 137 gigs. I think it also depends on the version of Win 98 that you are running. You should be able to partition the new drive into smaller partitions that Windows will see. I'm pretty sure that most hard drive manufacturers still offer a way to do this while installing the drive, but I may be wrong about that since Microsoft no longer supports Win 98. If the manufacturer did not offer this, I believe there are some third-party software options that will allow you to partition the drive.
You've installed 2 Operating systems on your hdd. You might want to back up ur system and do a clean install.
You could make a separate partition for Ubuntu, so creating a dual-boot. Or try the easiest method of using WUBI to install Ubuntu as a Windows file. This allows you to use Ubuntu without affecting XP or the hard-drive. Type WUBI in a browser and follow the instructions.