back up all the date
Check the HCL
Safety.
To backup all important information, check the hardware for xp compability, check drivers for the hardware which are compatible with XP.
the answer to that one is prob. in your textbook
BIOS virus detection functions
No you should not buy softwares before you buy your hardware because buying hardware first helps you decide what softwares will be compatible with your PC or notebook and then see which ones to buy.
Prepare everything you need
Vista should run the latest games with the proper hardware so it should be able to run Call of Duty 2, of course you may not have the required drivers or hardware and may need to upgrade.
If the hardware is compatible and you have enough memory you should be able to purchase an "Upgrade" version of XP.
Yes, you should always be sterile before performing any experiment as to not add any variables.
In theory you should not have any problems but back up your stuff before the upgrade.
Verify that the system meets the minimum and recommended requirements. Verify that all installed hardware components and software are compatible with Windows XP Decide how you will partition your hard drive and what file system you will use For a network, decide whether the PC will be configured as a workstation in a workgroup or as part of a domain. Make a final checklist to verify that you have done all of the above and are ready to begin the installation.