What changes have been made since the last known "good performance." ?? I would start by uninstalling whatever the answer is. Does it still occur??
If not, there is your problem. If so, you have narrowed the search criteria and can move on to the next thing.
When you say your screen goes black, do you mean that literally? For how long? Does it come back? Does your PC lock up? What applications are running while you are having these problems?
If you are seeing the hour glass frequently, then something is eating up your system resources. Have you done a Anti-Trojan/Worm scan?? If not, do one. You can download a number of free programs from
http://www.download.com if you don't have one already.
Glad to help out if possible, do the things/answer these questions mentioned and post a follow up.
Good luck.