Often times, for me at least, I have to launch a process which consumes a lot of processing power, such as archiving a large database or compressing a large file. These processes use so much of your CPU that your computer is practically unusable until it is done (if you have a single core processor).
Simply change the priority this process has by doing the following:

1. Enter “taskmgr” in your Run dialog
2. Select Processes
3. Right click on the greedy process
4. Set Priority to BelowNormal or Low

Since pretty much any process you launch normally will run will with normal priority, they will get the CPU first and your greedy applications will get what is left over.