I fink it depends what you are saying ...
is it ... 5 year old pc is slow (and we all say ... yes they are!)
(more memory may help 512 to 1Gb is plenty - don't go mad)
or are you saying that it's slowed down of late?
if its just started to go slow
windows likes plenty of space ... save all your music photos etc to CD/DVD and
download
http://www.ccleaner.com/download
run with the defaults
then
run the disc checker and finally defrag (both in disc properties (do it for all your discs)