As said, you're actually asking the wrong question; what you want to know is "My Mac seems to be running slowly, and is lagging when I type--what might cause this?"
It's exceedingly unlikely it's a virus (download and run ClamXav if you really want to prove it to yourself), but if there's lag typing obviously something isn't working right.
Searching the forums will probably give you a dozen suggestions, but the first thing to ask would be:
1) Is it lagging in all apps, or just certain ones (only Word, only Safari, etc), and if so, which?
2) What applications do you have open when it's slowing down? (Also, any web browser windows with Flash stuff in them while it's misbehaving?)
3) Have you installed any system tweaks or hacks?
4) How much RAM do you have?
5) Open Activity Monitor and see what it says current processor useage is--if it's near or over 100%, sort by CPU and see what the top culprits are. That's most likely going to identify your issue.