Hello, welcome to the forum!
It might be because the motherboard won't support it, don't quote me though. I'm pretty sure that the 4350 is a PCIe 2.0 GPU, but the motherboard specs don't state that the motherboard supports PCIe 2.0 based on the data sheet that I'm looking at (http://h18000.www1.hp.com/products/quickspecs/12546_div/12546_div.HTML), looks like that model HP was out in 2008, kinda old.
The best advice would be to see if the card is good by putting it in a newer system and if is good you may want to try reformating your HP and try a fresh install of the OS.....no promises though.
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Requiescat In Pace AMD Processor Forums | Member since: 1/19/2009 | Post count: 4142
Please don't PM me with questions, instead create a thread so that everyone can assist and benefit from the knowledge provided. Thanks in advance!