How to increase scan speed?

Antivirus programs take a long time to scan in general. If you have a faster hard drive that will help, as it is very heavy use on your disk. Deleting excess files, cleaning out temporary files will help alot to.

Also only run one scan at a time, doing more than one at once will more than double the scan time.

Cleaning out your system by removing old/unnecessary files will help. The less files it has to scan, the shorter the time it will take. CCleaner is a great program to do that.

As for the scan itself, The processor would be my first upgrade and hard drive second.

What is your current system? Upgrading to a dual or quad core will help if you don't already have one. A quad core is recommended. This will allow your computer to be usuable during a scan without slowing things down too much. See this Tom's article. This page ibenchmarks compressing a file while running a game. You can see that the dual core processors suffer a much larger performance hit than the quad cores.
http://www.tomshardware.com/reviews/phenom-ii-x6-1075t-...

Also this page from the same article shows the scan time of a dual core vs. a quad core. There is quite a difference.
http://www.tomshardware.com/reviews/phenom-ii-x6-1075t-...

my computer specs is
Quad Core Q9300 @2.50GHz
3 GB Ram
120gb samsung 7200rpm hd

what should i upgrade to make the scan faster?

Best answer selected by Aztec7.