Mac Pro strange powering up issue

So a bit of history.

Woke up one morning (as the song goes) and my 2007 Mac Pro would not start. No sound or glimmer of any kind.

After snapping off the battery clip trying to replace the PRAM battery to see if that had anything to do with I broke down and took it to the repair shop.

There they diagnosed that as suspected the power supply had croaked.

They could replace it no problem.

They also said a bit of antistatic tape would hold the PRAM battery in place. This they could also do.

I have just got the machine home and two things:

1) The machine starts up as soon as I plug it in. Didn't press the power button it just came straight on. It has done this subsequently. Also when I shut it down it come right back on again.

2) The fan now sometimes times come on high and stays on. The machine freezes when this happens.

Before I take it back to the repair shop does anybody know what might be causing this so I have some knowledge when I talk to the guys.

Many thanks.

A.S