My computer has been too slow to open and close the programmes. I downloaded the PC booster and PC velocity but still the problem is same. I have got Norton Anti-virus as well but it didn't find any virus. What would be the problem? Is it necessary to reboot?

There are many possibilities. You have two tools built into the system that may help: disk cleanup & disk defragmenter. Both found at the end of this path:

start > all programs > accessories > system tools > disk cleanup

run this then the same path to run disk defragmenter.

rebooting is good!