You do not mention the Op Sys or how much memory you have - some say that more memory will speed things up but there are things you can easily do before adding memory.
Get (download) Microsoft's Windows Defender (free) if you don't already have it.
Use the Software Explorer feature to prune rubbish from the Start Up Programs list - best to leave in any security stuff .
Use the same programme to examine Currently Running Programs and permanently delete any that look like malware.
Get Spybot Search & Destroy (free) and scan your machine for spyware.
Get Piriform's CCleaner and use its Registry utility to tidy up your machine's registry.
Defragment the disk(s)
Do regular Housekeeping to keep the machine well oiled.
Then if it's still running too slowly consider more memory.
Peccavi
February 2009