Computer locks up and sound looping.

Well it's been a long time since I bothered to try and sort this problem out since every time I ask on forums like this, I get the same answer. 'Its the psu'
I have a 450w Corsair psu, which should be able to run on my computer according to a couple of good search's I've done and also according to someone on this forum when i was thinking of getting a new graphics card.
Right the problem is in the thread title. When I have the graphics card plugged in, my computer locks up (not being able to move mouse, alt-tab, or ctrl-alt-del). I normally have to restart my computer. And it doesn't just happen in games, it happens whenever it wants. When browsing even when sitting and doing nothing.
My computer has also started playing up without the graphics card in but when trying to upload something to a file sharing site like rapidshare or megaupload. I noticed it about a week ago when I uploaded something to megaupload. It went at 20mb/s for couple min's then back to 80kb/s which is my normal speed. I thought it was my ISP testing speeds. But I tried to upload something again couple days later and it was 20mb/s again, so i tried to upload to rapidshare and it locked up my computer. I then installed Windows 7 to see if it was a bad OS, but the problem was still present and it now locks up when uploading to megaupload. Its only my computer in the house that does it. And its connected wirelessly. I have also taken out the network card to see if that was the problem, but megaupload still uploaded at 20mb/s and rapidshare just made my computer lock up.

So basically, What the hell is wrong with my pc