Links

These are utilities that I found to be important and safe to recommend.

Codec Sniper [download]
Repairs and detects problematic codecs and directshow filters. No need to install - just execute!
Use this application if you are having problems making video reencoding on the fly to work.
Repair MDAC / Jet components utility from MacroPool [download]
Repairs the Microsoft Data Access Components (MDAC) and JET drivers already installed on your system.
Sherlock - The codec detective [download]
Shows information about installed video and audio codecs
MPEG4 Modifier [download]
Enables you to quickly remove the "packed bitstream" from an AVI file.
K-Lite codec pack [download]
A good collection of codecs, frequently updated, stable, uninstalls non-recomended codecs and dshow filters.

Info: The above utilities do NOT contain any spyware, adware, viruses or any other software similar, but use it at your own risk