Hello there
I would recommend sticking with XP for the moment, Windows 98 actually has a lot more attacks from viruses and things. (or used to)
The best thing to do would be to Boot off the Windows XP CD, partition and format the drive using NTFS.
Once windows has been installed, load up your antivirus software straight away, if you don't have a copy then download AVG Antivirus. (Make sure you update it as well)
http://www.pcworld.com/downloads/fil...RSS,RSS,00.asp
The next thing to do is install the windows service packs, so just connect to the microsoft update site and get those downloaded and installed. (make sure you are running Service Pack 2 by the end of it)
http://www.windowsupdate.com
Once all this has been done it would probably be a good idea to install a spyware program e.g. Microsoft Antispyware.
http://www.pcworld.com/downloads/fil...RSS,RSS,00.asp