Microsoft has the idea that WMP doesn't have to be optimal in regards to multitasking. If you find a cure for that, let everyone know.
-> But I will share I use Winamp, Media Player Classic instead of WMP since they have a lighter load on the CPU. Both have free versions.
Bob
hi,
i've recently been having problems with running movies on my computer (Windows XP/SP2, Pentium 4 CPU 2.8 GHz, 448MB RAM). The problem always arises when i'm running a movie on Media Player V.10. At first the movie runs fine for a bit, but then my machine begins to slow down, with the video lagging behind the sound. i've checked the task manager, and it shows Media Player using 100% of my CPU. I've done a number of things to fix this problems. I've installed spyware protection programs, anti-virus protection programs, windows securities updates, cleaned up my desktop, removing any unnecessary programs at startup, clearing up the bugs in my computer's registry files, even defragging my computer. yet the problem continues to persist. if anyone could give me any advice, i would gladly appreciate it.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic