I have played with computers self teaching myself how to build, upgrade and troubleshoot myself for many years now, it has also helped when times were tough because i was always able to bring home a few "strays" as my ex called them to repair or upgrade
a few things i have noticed on nearly every PC i have ever worked on usually due to "speed' issues was all the start up programs running when you don't want them to run
here is a tip you can do that will speed up your system drmatically..
go to RUN and type in msconfig/enter/startup tab and uncheck everything but your av (or any programs you don't want to run at start up) my guess is you will have 50 programs running in the background
for win 98 and older you will need the 'sys tray" checked
you can also go to RUN and type in %temp% and delete everything in this folder, not everything will delete the first time because your PC is using some of these files, but after a reboot and doing it again everything will delete
Hope you find this usefull