Problem while attempting a factory restore?

Ok, here's the deal:
I have a Dell XPS M1530 Laptop from 2008. I mainly use my Mac for school and keep this laptop for gaming. I was really deficient in using virus protection software and as a result it's completely overrun by viruses/malware. I followed the malware removal instructions on this forum, but I'm sure there's still malware in it. It's also got a lot of random stuff on it that I don't use anymore. Since there's not really anything I care to save, my plan is to do a factory restore.

However, I run into a problem when trying to do a factory restore. I press F8 as my computer is starting up, go to repair my computer, and then it takes me to a screen where I have to log in. The problem is that the only option is "other user", and I've tried every combination of my username and password to no avail - it won't let me log in.

Any recommendations? Also, is there any good reason not to do a factory restore? I really don't care about backing up any of my save files, etc. - at this point I'd much prefer a fresh start. Thanks for the help.