My computer is running slow ?

i hav a desktop of 1gb ram ,320hard disk(sata), core 2duo processor,intel orignal motherboard.m using xp2 OS. previously it was working very wel bt now it is slow when i want to copy anything in pendrive it copies very very slow......even if i want 2 format the pen drive it denies to formate . it is not wd the same pen drive ,i used several pen drive it replies the same..............so plz help me how can i mak my computer faster as before.m using kaspersky pure anti viruse trial verson ...which anti viruse is best to use........plz suggest