Okay, I was on my computer from 3:00-5:30PM today, just doing normal stuff. I log back on at around 7:15PM, and the background is blue, with a warning message in the middle saying the computer is infected with Spyware. The new screensaver is a bunch of bugs running around the screen. Any attempts to change these work initially, but are reverted when I log off. How should I deal with this problem?

Also, as a side note: The said computer has had lots of problems these past months, starting all the way back around September, maybe earlier. We’ve had a computer technician come over multiple times, but what he installs only seems to, how should I say this, cut off the tumor, but not treat the source. If, hypothetically, I wanted to revert my computer to how it was when we originally got it…would this, first of all, be possible? Would it cost more than just getting a new computer? Sorry I can’t offer technical details yet, but if they’re necessary, I’ll try to get them.
Thanks!
The answers I’ve been getting are incredibly helpful. I still have to check with my dad and brother if they think it’d be a good idea to format the harddrive, but it sounds like, at this point, it’d be a worthy cause.

Keep the good answers coming; I’m going to extend the life of this question to milk all the information I can from people, as I’m learning a lot! Thanks again!