It's always good in Window's machine's to frequently check your startup folder (start>programs>startup) to see what program's are starting at bootup. That way, if you have a trojan or something strange in that folder, you could simply delete it. I know one time my neighbor had a trojan on her machine and she didn't know. When I went to the startup folder, I saw a program there called xplorer.exe which obviously isn't a program that should be there. I downloaded a trojan scanner for her and scanned it and sure enough it was a trojan horse program.