Vinia, I'll also give you a few simple, basic maintenance checks that you can do, to help make sure this isn't something hanging at the hard-drive level:  the boot-up freeze and the shutdown can also occur if there's something wrong with the HD, so if you do these two things you can at least eliminate that as a possibility.  The first two are "set it up and walk away" type tasks:  no babysitting required!

1) Defragment.  I'm sure you know this, but if your drive is heavily fragmented, sometimes it might take a second or even third pass to cut down on all the red in that readout.

(and in case you don't know this:  Start-->Programs-->Accessories-->System Tools-->Disk Defragmenter.  Click 'Defragment'

2) Run a low-level Scandisk.  Easiest way to do this is, in My Computer, right-click your hard drive, choose "Properties", then the "Tools" tab, then "Check this volume for errors."  Check both the checkboxes in the dialog that comes up, and when you click OK, Windows will pause a moment, then say something less funny than (but similar to), "Yo!  I can't do this right now!  You wants I should wait 'til you restart the computer?"  Say 'Yes'

When the machine reboots, it'll run the ScanDisk operation, and believe me, it may take a while.  If it hangs on one spot for more than, say, 30 minutes, you may need to (groan) hot-button the machine, resetting it so it can take another stab at it.  If it gets hung several times, skip ScanDisk and just post here that that's happened--I can help you turn that process back off again.

99 times out of 100 though, I don't have a bad machine hang up like that, so you should be fine.

3) Make sure you run more than one anti-spyware tool.  Keep it updated weekly, run scans weekly or bi-weekly.  I use Ad-Aware and Spybot.  One often catches things that the other does not.

4) Check to see that you have at least 25% of your hard drive space free.

Hope this is helpful!

Glad to hear you have a partial success!  Hopefully your upgrade will stabilize things even further.

Here's a good article on upgrading the PSU, in case you'd like some info on choosing the right one:

http://blogs.zdnet.com/hardware/?p=307

And a link to the Power Supply Calculator at Antec's site.  Should help you figure out the size you might need:

http://www.extreme.outervision.com/powercalc.jsp

Good luck to you!

In the first instance I upgraded the soundcard drivers and started the computer a few time over the course of a day or two. The halt occured only once during the first boot immediatly after installing the latest drivers, subsequent times It didn't halt on boot or shutdown. I took the opportunity to scan the HD's a couple of days later but no errors were indicated or at least nothing untoward presented itself.

Thus I'd say that the driver update was the solution, but my PC is probably better for the scan and would be even better with a new PSU. Once again thanks all!