Hi peeps, just a quick heads up about this.
I had started to experience crashes and lost recordings on running media centre both through vista and my xbox 360 as an extender. Looking at it in Vista I found all of them to be ehshell.exe crashes.
After some searching around the internet I found the problem!
Apparently sometimes the files for the EPG and associated files start to get really long file names (some over 500 characters) and this causes the crashes.
The fix is simple. Find the EPG folder (I used windows search, advanced and show all files inc hidden etc) and delete it. Then reboot MC, scan for channels and then download the EPG. This sorts it out. You will have to reschedule all of your recordings though.
I hope that helps someone!