People will say "add more memory" and they may be right but there are things you can do for free before you need to spend money on memory...
Download for free the following three applications...
Microsoft's Windows Defender (you may have this already)
Piriform's CCleaner
Spybot Search & Destroy
Use the Software Explorer feature of Windows Defender (Tools) to clear rubbish from the start up list - over time naughty programmes get themselves into your machine and make themselves an entry in this list and consequently load at start time without your knowledge.
Run Spybot and get rid of any Malware it finds. There will probably be lots of stuff the first time you run it.
Run the Registry-tidy-up tool of CCleaner.
Defragment your disk(s)
Use the Currently Running Programs feature of Windows Defender to see what's running in your machine - make a note of anything suspicious and use add/delete programs to get rid of them.
Run Spybot weekly. Clean the registry and defrag the disk(s) a couple of times a year.
At any time you can right click on the Taskbar at the bottom of the screen and select Task Manager and view system performance and which processes are eating resources. At the moment my CPU is running at no more than 5%.
Good luck...
Peccavi, April 2009
