There's a lot of hysteria associated with SP3, so I wouldn't take what you've heard too seriously.
rthdcpl.exe is a process belonging to the Realtek HD Audio Control Panel and is bundled alongside Realtek sound cards and audio hardware. "This program is a non-essential process, but should not be terminated unless suspected to be causing problems.\r" We strongly recommend that you run a FREE registry scan to identify rthdcpl.exe related errors.
It's part of the RealTek Audio Manager. Note it's not the audio driver itself, so it's not really necessary for your audio to work.
First thing to try is to uninstall it, then reinstall. Can you find anything related to "realteck" in Start>Settings>Control Panel>Add/Remove Programs ?
If you can't uninstall it first, the next thing to try is simply reinstall on top of the old install and let the new one overwrite whatever is wrong with the old.
Check Control Panel>System>Hardware>Device Manager>"Sound, Video and Game Controllers" and identify which RealTeck audio device your computer has (usually it's AC '97)
Whichever one it is, download the corresponding software from this site:
Save it to a folder on your desktop, and run the software.
Reinstalling the software should fix your problem.