virus issues

i have a dell inspiron 9300, usin window xp and pay as you go internet. for the last few months i have been expierencing some problems with my comp, my conp freezes, crashes, closes its self down. my firefox shuts down without warnin, i get non responding scripts constantly my hard drive makes some strange noises all the time and my email has spam. i hve anti virus that i password protected but when i try to type in the password it tells me its incorrect. im really stuck on wht to do as when i run a scan it picks up the same result everytime thats when it does manage to scan as ive been havin alot of problems with that too. im really stuckk and in need of some help plz