Start - Run - Msconfig
Go to the startup folder and remove (untick) all the shite that doesn't need to be run on start up and ok it!
When you close it, select exit without restart for now
Right click my computer, properties, advanced, "perfomance settings", advanced, change Set both your initial and maximum about 500k more then what is already there e.g if it was:
Initial size (MB): 1500
Maximum size (MB) 3000
Then change them both to 2000 and click "set" and ok it.
If you're not so fussed about fancy graphics and so on, in the visual effects part of the Performance options you can change it to run for best performance instead of best appearance.
Go in your add/remove programs from control panel and remove all that dodgy crap you have installed and never use.
Start - all prog - Accesories - system tools - disk cleanup,
Remove the crap you have in there (minus the hibernation files)
then finally go to start - all prog - accessories - system tools and this time defrag and set that off!
Once it's finished all that reboot your machine and it should run better for you!
Also when you reboot, tick the box that comes up and ok it. It runs the first time after a reboot when you change anything in Msconfig.
Hope that sorts you lad!