A few months ago I had to perform a clean install when I updated from Windows XP to Windows 7. I had hoped to be able to simply upgrade from one program to another, but my computer had been corrupted by several very annoying viruses.
I took my computer to a technician and he said if I did a Windows 7 clean install it would cure my viruses and leave me with a faster running computer. He was right. I am now virus free. Lets hope things stay that way!