Press Ctl/Alt/Delete to open your Task Manager and end the process of this program which is using your cpu. Then try to track it down and delete it.
Some programs you d/l off the interent have bugs imbedded in them, such as the 50% and 100% Cpu Usage Bug (which I d/l from Guru's Ray Adams Ati Tray Tools, which will install the 50% bug and is hard to find. It is inserted everywhere in your registry). The 100% bug is often included in some Microsoft d/l, especially if you use Internet Explorer.
Vista also seems to have a memory recovery program which runs for the first 10 minutes after bootup, incrementally reducing the ram allocations to the dozens of useless programs that Windows loads, but you don't need. This program usually only uses about 5% cpu, but is annoying if you have a noisy hd like a WD.