Helpful Links Regarding Malware Protection
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.
See these Apple articles:
If you require anti-virus protection I recommend using ClamXav.
It's very unlikely your problem is a virus or malware.
Upgrade your RAM to a minimum of 4 GBs, if possible. Upgrade OS X to Mountain Lion, but not until you find out what your problems are or if you do an erase and install.
Is there a way to scan my Macbook Pro 13" Mid2009 for a virus?
As long as your software is fully up-to-date and you aren't running Java 6 then you are fully protected against all currently known malware, so as Kappy already said, it' unlikely to be your problem and you should not waste your time looking for it with scanning software that will just slow it down even more.
I'm constanly getting the spinning ball - I've taken it in to the Apple store so many times in the past - finally got it upgraded to 2GB the /day/ my warranty was ending! - running OS X 10.7.5
Your MBP can hold 8GB and although you can technically run Lion with 2GB you will always see the beachball if you have more than a couple of apps open or a bunch of tabs in your browser. I suspect if you open the Activity Monitor app and check the memory tab you'll find your problems there.
Here are some other ideas from Randy Singer Macintosh OS X Beachballs!.