While using on board graphics go into windows and install latest AMD drivers. Then power down switch the monitor to the GPU and see if it works. If not the problem may be within the PCIs slot or GPU. I assume you connected the two 6 pin power connectors right? What type of power supply do you run?
Power supplies will go bad with age due to capacitor aging
"The capacitance of certain capacitors decreases as the component ages. In ceramic capacitors, this is caused by degradation of the dielectric. The type of dielectric, ambient operating and storage temperatures are the most significant aging factors, while the operating voltage has a smaller effect. The aging process may be reversed by heating the component above the Curie point. Aging is fastest near the beginning of life of the component, and the device stabilizes over time. Electrolytic capacitors age as the electrolyte evaporates. In contrast with ceramic capacitors, this occurs towards the end of life of the component."
since the PSU is from 2002 according to OP I would really replace the PSU with a modern one
Antec,Seasonic,Corsair are three brands I would recommend
I personally went with a Antec High Current Gamer Series 520w myself
the Ante HCG series are a good lower cost option
My recommendation is to replace the PSU with at least a 500w newer one
Any PSU from 2002 is suspect and TBH Thermaltake doesnt have the best reputation for making PSUs
I am going to try it in my mates pc later, he has an *shudders* ocz power supply, but it is pretty new and is 550w so should be fine Thanks for the help guys, if it still doesnt work I will post a reply
But is there a chance that using my psu could have damaged the card?
One thing i am confused about is that surely when the pc is booting up the card is running at idle thus consuming only about 150w so why wouldnt it work? I have been running a 5850 for the last few months
Since you say the PC boots and uses the onboard video even when the HD5870 is installed, does it show up in Device Manager?
You say your PSU has a 20-pin power connection...yes, supposedly those work in a 24-pin board, but I am wondering out loud here if the PCIE slot isn't getting [enough] power. This is another reason to replace that PSU.
Test it in your mate's PC. At this point, it could be a bad card. If it works in his PC, then it comes back to a possible power issue. I assume you aren't hearing a loud beep, since I'm sure you would have mentioned it; more like a scream, it would be the graphics card moaning that it isn't getting enough juice.