Sounds like a memory leak problem.
How long has the computer been running without being rebooted? When you try to open a program, can you hear your hard-drive thrashing? Once you run out of physical memory (your DRAM or SRAM), the computer uses the harddrive to store process data -- which is in the magnitude of thousands of times slower than main memory access.
So most likely, your computer has been on for a very, very long time. Windows is notorious for slowly leaking memory and never collecting it, meaning that eventually every process run will be allocated space on the hard-drive, which will cripple your computer's performance.
Try rebooting every couple of days.