"The antivirus stuff pops up asking me to buy more software."
Does it show a name, like, Vista Antivirus 2012, etc.?
Also what Operating System are you using, Windows 7, Vista, XP?
Do you have access to any clean Windows computer?
If you could give us more info, it would help.
In the meantime, try the following:
Start your computer.
Tap the F8 key before Windows starts, to bring up the Windows Advanced Options menu
Use the arrow keys to select Last Known Good Configuration (The most recent settings that worked)
If after several attempts you are unable to get into the XP Windows Advanced Options Menu, as the computer is booting into Windows, turn it off pressing the on/off button on the computer case for about 4 to 5 seconds.
Now, turn the computer on, once again.
Windows should notice that the computer did not successfully boot the last time, and show the Windows Advanced Options Menu.
From the Windows Advanced Options Menu, select the Last known good configuration. (Highlight and press enter to load)
The computer should restart and load into Windows.
Let us know how it goes.
Retired - Doin' Dis, Dat, and slapping malware.