My iMac is acting up in an annoying way.?

My iMac which is a 20 inch probably late 2008 model or 2009 (not sure) recently is acting up. My keyboard which is apple's sometimes capitalizes all my words as if caps lock is on which it isnt. and my mouse scroll button wont work. After a minute or two of typing randomly and either holding the shift key or turning on/off caps lock, the mac goes back to normal. any suggestions what is wrong with my computer. i could reinstall leopard fresh, but don't want to have to backup stuff. but if i have too i will