Sudden PC slow down

Hi. Long story short: my PC started to drastically slow down after I soft-reseted it after it hanged while watching some anime. While I have encountered this problem twice in the past, the only solution I've resorted to is to reformatting my computer. I'm not sure if its the processor or the RAM, but it is highly unlikely to be a virus (I scanned my PC on the same day, probably an hour before this happening, and I scanned it afterwards, no virus attempts).

Details on the "drastically slow down" part: the loading screen with the "Windows XP" logo takes as long as seven minutes, start-up itself takes another five to seven minutes. Games and applications take as thrice as long to load, and the lag is bad as well (1 Gig RAM). While I use the Norton Optimization feature frequently (even before this happening), it did very little to help.

Hope anyone can help!
-David S