Just installed snow leopard, now time maching stops on "making disk available"

Just installed snow leopard and now my time machine won't back up. It only gets as far as "makign disk available". I can go into my time machine and see the whole history of updates. Last successful update now Feb 9th! If I try to start a back up I can't then turn the Mac off without holding power button. If I look on the help website it says time machine may take longer if:

Long time since last back up - Feb 9th I gues counts

If Mac not turned off proerly in meantime - Yes, everytime I've tried this

If just upgraed software - Yes to snow leopard

I've even paid to have it looked at in a Mac support centre where they backed up onto a different hard drive and then did a clean load of snow leopard for me

I'm at a complete loss. Never had any probelms with Mac until loaded snow leopard. Now I have time machine problem and continual crashing of microsoft office for Mac 2008

Help. please...