Help! Computer Will Not Boot Up

[Slowhand]

I have an old HP 6340 It has an Celeron 333. I purchased this item new in June 1998. Over the years I have added a Maxtor 48 Gig hard Drive , a video card, and Soundblaster 5.1 sound card. Also added to sticks of memory to bring it up to 256k. I also went from Windows 98 to Windows XP Pro. It was always kind of noisey booting up and even running programs. Until yesterday I never had a problem with the system. During the afternoon it just shut itself off. There were no line surges or spikes while is was home. I looked at the Surgemaster connection and nothing tripped. The power lights to the printer and monitor were still on.
So I turned the system on. I heard the typical whirling and clicking sound it made when rebooting, however thats all it kept doing when I came back 10 minutes later thats all it was doing. At that point I sat there and watched for a few minutes more but nothing. So I decided to shut it off, but it would not shutdown using the on/off switch it just kept whirling and clicking. I would appreciate any replies to what is going on. I am a novice as far as technical issues go. Thanks in advance!

[muno]

Ram is counted in megabytes (so you probably have 256mb).

Often in bios there's option 'soft off' which effectively selects whether you need to push the power button for 4secs or just click it to shutdown.
So, try and press the power button for 4seconds and see if it shuts down.

Whirling and clicking sound isn't healthy, whirling sound comes usually from dusty/bad fans. Clean them up, or replace them.
Clicking sound could come from the HD, which in turn would mean the hd is bad.

Do you see anything on the screen when it tries to bootup?
-M

[Micro Bean]

From what I know HP doesn't give you alot of wattage when it comes to PSU. Normally they supply you with 125 watts or a little more maybe it could be the power supply. Since you added more stuff to the computer the PSU cant handle it and yes I agree with dust and junk that builds up on those computers. and also check whether the fan is running on the cpu. I think your computer is heating up and shutting down. Maybe someone else can enlighten on this situation.