Could be that your resources are low, Right Click the My Computer icon on your desktop, then L/click properties, then prefomance. Should list system resources. Check it with Q/B open and closed.
To free up resources check;
http://www2.whidbey.net/djdenham/Running_items.htm
To free up resources check;
http://www2.whidbey.net/djdenham/Running_items.htm