The suggestion of a wrong - or corrupted - Codec sounds most reasonable. Note that Foostroupe suggested he had the right Codec for general use, but a few things played back wrong. Your friend may have the "wrong" Codec for general use, but close enough to play everything wrong. This could happen through an inadvertent change in setup, or through corruption of an existing file.
Some versions of Real Player have a known security hole, and patches are available for the versions affected. If your friend hasn't checked to see if he needs to be updated, he may have acquired this "feature" by being attacked by someone attempting to use the hole. There have been very few reports of attempted exploitation of this particular security defect, and even fewer reports of successful attacks; but an attempt may corrupt a few files even if not successful. (Similar holes have been found in other popular "media players," and an attempted attack on one of them may have corrupted a Codec used by Real Player.)
There are numerous offers on the web for "utilities" that promise to "improve your performance." Those of us who lived with earlier Windows versions, and occasionally found some utilities helpful, might be tempted to try one. With XP, it is almost safe to say that NONE of these will be helpful, and the majority will be harmful. Many such programs are actually malicious programs to put embedded spyware on your machine. WinXP is arguably the first, and only, OS that Mickey$oft has produced that doesn't need any such "help," if you're willing to learn how to set it up to suit your personal needs. If your friend has permitted ANY such downloads, they are a suspected cause of the problem.
In ANY CASE of unexplained changes in performance, virus or other "malicious external attack" must be suspected. The other cause is almost always "messing with the system" instead of learning how to run it.
With WinXP, a systematic fix should always start with:
1. Use regedit to manually back up your registry. 2. TURN OFF SYSTEM RESTORE to avoid "restoring" a registry copy that's already infected with something. 3. Go to any reputable AntiVirus makers web site, and get your machine scanned. 4. Use any of the available AntiSpyware programs to scan for any such infection. There are numerous ones available, but AdAware and Spybot are a couple. If possible, use more than one, since none of them can detect all known infections.
Once the above has been done, and any problems there have been found and fixed, you may want to turn System Restore back on. The present case suggests removal and reinstallation of all the Codex files. This should be fairly simple to do if your friend has his installation CD. Codex files are also generally downloadable from Microsoft and/or numerous other sites.
With respect to changes in clocking: Overclocking is relatively simple to do. This means running the system and/or processor clock(s) at a speed higher than intended. It is routinely done by some people, particularly by those who build their own machines; but since it makes the processor run hotter, it will usually result in smoke, flames, and PERMANENT hardware failure if done on a system not specifically designed to provide the necessary additional processor cooling. Used knowledgeably, it should not produce the indicated behavior.