Guys!
I found this nice trick and it'll help you in improving system performance of your Vista. I hvnt tried it in XP.
1. Start -> Run ->
msconfig2. Goto "
Boot" tab and click on "
Advanced options..." button.
3. Now enable the option "
Number of processors:" and select the correct no. of processors from drop-down list.
4. Now click on "
OK" button and then "
Apply" it.
5. It'll require a restart and after reboot you'll notice improvement in system performance.

Tell me whether it worked for you or not? I noticed a major improvement in my Vista.