My computer’s applications keep on closing on their own. why?

can someone please help me? i have a dell dimension 4500 with a 256mb ati radeon hd 2400 pci card, 1024mb Ram, a 40gig hdd,and a 350W PSU. lately, when im using a program, randomly a window will pop up and say: “(insert program name here) has experienced a problem and needs to close. would you like to send a error report?” it keeps on happening more frequently, and i have run virus and malware scans, but nothing has turned up. i am going to try a windows xp repair disc, but what might be wrong with my computer? can someone please help me? and fast?