Hmmm - I am following this thread because I am in the same boat with D2. I am not hijacking this thread, just adding symptoms because I think this is the beginnings of a common problem. I have seen other similar reports on other forums, none resolved.
In my case:
No recent HW or SW (including drivers) changes - outside of security updates.
XPPSP2
3.06GHz P4, 1Gb RDRAM
WMP10 and 11
Lots of free disk space
CPU usage normal
Affects all sounds from all sources - MP3, WAV, WMA, etc. - including system sounds (Windows Startup, empty trash, new mail, etc., plus the various clicks and dings), streaming Internet radio, and audio CDs
Slow down is just that - a four minute song will take approximately 4 minutes, 40 seconds to play with sounds slowed to where all pitch is lower (like putting thumb on a record turntable - for those of you who are old enough to know what a turntable is.

)
And to make troubleshooting harder - the problem is intermittent - with no apparent trigger bringing it on, or any particular action to make it go away.
Things I have done to resolve issue:
Complete malware scan - AdAware, SB S&D, AVG Anti-Spyware scan, ZoneAlarm Pro AS Scan, HJT (and yes, I am qualified to analyze HJT logs), rootkit scan, TrojanHunter scan.
Moved M-Audio Revolution audio card to new PCI slot
Updated drivers
re-installed WMP
@D2 - if the above does not fit the description of your problem, then I apologize and will immediately back out of this thread, but keep watch.