I use spybot to get rid of some spyware (not one program will do all)
and this will also check your registry(yuk).
usually if I download or install a new program/update, I defrag.
These two methods usually keep my pc(yuk) running responsively.
After time as gone by I also right click the system disk for properties and
do a disk clean(without compressing to save space) and also
get rid of all but one of my restores.
Also if you clean out your prefetch drawer this can help
(Spybot I think does this also).
I have gone previously about 5 years without a re install
and think spyware and fragmentation is the cause of slowdowns,
also registry rouge entries through faulty unistall/install
scripts can {bleep} up a good pc.
Ps This is my very first post sent via AWEB on the Amiga
(usually I use my PC)