disk recovery Follow

Hi all. I have a dilemma.........I formatted my parents computer in April due to a virus. Here it is 5 months later and I have to do it again. Actually, it seemed like it wasn't running like a 'new computer' after that format; not as quick, and they usually do after a reinstall....One thing that was really weird though AFTER the format was - before i formatted i installed the kapersky antivirus. We were using the F-Secure antivirus through our ISP but I had heard some good things about kapersky but it didn't help. HERE'S the weird thing. After the format, and things just weren't quite right, Kapersky started showing up in the computer and I have NO idea as to how that happened. Now, i formatted using the recovery partition like I had done so before. Now it needs to be done again.....and I don't know if this is going to work RIGHT this time or not. I am wondering IF there is a difference between formatting using a recovery partition? OR by using the Windows XP Disk? What would be the best? Also, is there anything I can do - by keying in certain codes or whatnot (i had to do this once with a dell computer I had - tech support walked me through it). I really don't know which route to go and could really use some help Thanking You All in advance, Karen