completely lost

so i downloaded firefox because i was the only kid that had avant....

i never had any problems before and now i have popups like crazy(WINFIXER) and sometimes when i leave firefox open at night, my computer automatically restarts and i get a dialog box that reads "windows has recovered from a serious error"

i run microsoft antispyware, spybot, and ad aware all the time. cant seem to get rid of the problem.

please help.