[mischievious]

It sounds like you may have more than one issue:

The "clicking" sound is likely down to the hard drive. if the clicking is lound and rythmic e.g. a single click repeating would likely be a sign of hard disk failure. If the clicking sounds like a bee in a matchbox (random but intense) then it could just as well be a sofware issue as hardware.

If the issue were the power button then it would still shutdown so I'm not convinced about that one. It would prevent power up though so it's possible that it's broken though not likely. The fact you powered up by holding in the button is intriguing as the reverse is usually true, hence my comment that it could be broken.

I agree with the last post concerning the event viewer, which "should" give an indication of why you couldn't shut-down. my money is on a process or service which is not giving up its resources. In both XP and Vista you can take a look by running eventvwr.exe command (actually runs eventvwr.msc). I post "should" as in these cases this is more "gives hope".

As to your query regarding cheap deals on towers it looks as though you really don't want to spend hours trying to fix the problem, personally and I know this may be considered controversial but I would suggest Dell. Depending upon your budget even if you buy a reeeeally basic box you get a cracking case.

In both cases though I would suggest copying any valuable data somewhere else somehow as the integrity of you drive has not been verified. chkdsk verifies the NTFS structure not drive integrity.