Black bars on side of open windows

I am running snow leopard and recently started noticing these black bars on the side of open windows...they appear when I have more than one window open. I have attached a pic to help you see what I mean. Does anybody know how to get rid of them.?

That used to happen on my MacBook Pro when I was running Windows XP and it was either a graphics card or RAM issue. Sorry I couldn't be much of help.

Well guys, it turns out that a failing hard drive was the cause of this as well as the cause of getting errors trying to copy files over to external disk. A couple of days after this post a new mac update required me to restart my computer and I got the dreaded circle with the slash through it on bootup. Drive not recognized. Good thing for the Time Capsule backups.