Pc problems and help

Hey guys I'm looking for some help and I'm wondering if this is the right place.
Since recently I have been experiencing a lot of problems with my Pc, problems including:

Lockups: Mouse and keyboard freeze up forcing a reset
Crashes while playing games: can only play for less than a minute then it instantly restarts
Crashes while playing movies and music: same as games.
Occassionally does not power up: takes several attempts.

I've ran virus checkers and scanners and it reports my system being clean, I'm pretty sure I have a psu hardware fault, I have had my pc for over 5 years now and it's pretty much showing it's age too.

Can you please help me and find out once and for all what the problem is?

Additional Spam:
whoops I put this in the wrong section.....my bad, could someone move it please?

Jam it back in, in the dark.

This definately sounds like a PSU problem, if virus scanners are proving your system clean. It would explain all of the above symptoms.

Just to be sure though, what virus/spyware programs did you run, and were they up to date?

Also, in the event that you need to purchase a new PSU, give us a rundown of your hardware so we might be able to give you some good advice.

There's nowhere I can't reach.

ok I used the latest version of AVG
heres a rundown of my hardware:

Msi motherboard with AMD athlon 1.3 ghz processor
160 gig hard drive
1 gig ram (2x 512 meg kingston sticks)
ATI radeon 9250 gfx card

another thing I noticed is that I can happily browse the net or use my IM programmes but when I want to start a game or watch a movie or listen to music ect, thats when the problems start.

This thing is sticky, and I don't like it. I don't appreciate it.

Just running the PC doesnt take much power. Basic functions still work great im sure. When you start opening programs that have to use the sound card or vid card (espically the vid card), or just more CPU, the failing PSU cant handle it, and it shuts off. I suggest searching google for "Sisoftware Sandra" and download the lite version. Run a full system scan. Once thats done check the bottom of the report and see if you are getting any voltage warnings. You may get something called a "V-Core" warning, which is just stating the voltage being supplied to the CPU's core is too low. This is also a sign of a bad PSU, or incompatible prosessor, but i doubt there is a prosessor issue. Hope this helps.

I am a dolphin, do you want me on your body?

Thanks but I didn't see anything resembling that, am I doing something wrong (since I have 0 knowledge on how to work that programme) or I'm just screwed?

I was speaking idiomatically.