If I try to play videos of any format the sound plays but the video is a still frame then the computer locks up completely requiring a hard switch off and restart. This has also now started to happen if I try to view them with Picasa.
I recently installed MAGIX PC Check & Tuning 2011 to try to speed up my computer and although it apparently found lots of thing wrong I haven't noticed any improvement. I'm not sure if I had this problem before or after I installed MAGIX.

Dp a System Restore to before you installed and used, see if that resolves the problem. Then throw it (MAGIX ) away.