1stly you want to do a virus scan with avg anti-virus,they are the best,no matter what anyone says,
2nd.you want to do a spyware/malware check.use avg anti spyware for this.
3rd.you want to run a adware scan.use adaware for this.

the above programs are free and tried and tested as i have done this over 20 times to pcs and laptops running vista and xp.

4th go to start,control panel,system,advanced,performance, and click on adjust for best performance.
this will make your taskbars etc go back to old school taskbars,but you can change them back without losing responsiveness.

5th then go to start control panel,add/remove programs,get rid of whatever you dont use.

6th go start,run,msconfig hit enter,then startup,untic anything which says software in the key which will be all of it,then apply.it will say restart computer but deny.

*there are also services that slow down your pc and enable access for hackers etc.on the majority of pcs i can get into it from the password screen in 5 secs even if i dont no ur password.no joke
i will instruct u by pm if you want to do any of these,its very beneficial for you but dont want anyone just clicking here and there and messing shit up and blaming me.

7th you want to go start,run,regedit,double click HKEY CURRENT USER,then CONTROL PANEL,then DESKTOP,the find menushowdelay and change the value from 400 to 0,then ok/apply.

after this,restart your computer,u may notice its slower loading up wen it comes back on,this is just the pc getting used to the new configuration.when the desktop loads up again it will have a box saying youve messed with a windows utility etc.just tick the box and ok it.then restart your computer again.
once it has started up again you will see a massive decrease in the amount of time your pc takes to do things.

THIS DOES WORK I HAVE DONE IT TO OVER 20 PCS AND LAPTOPS,ONE WAS A BRAND NEW £700 HD ACER ASPIRE 6930G LAPTOP FROM PC WORLD AND IT WAS FUCKED,RUNNING 1 PROCESSOR WHEN IT WAS DUAL CORE,AND 1 GIG OF RAM WHEN IT HAD 3 GIG.ONCE I SORTED THIS,AND DID THE ABOVE IT FLEW AND USED FAR LESS THAN VISTA WAS MENT TO USE ON IDLE.

any queries dont hesitate to ask.

hope this helps

jamie