Dell Inspirion 537

Ok, a friend wanted me to look at their dell tower, said they'd taken it else where and they wanted $$$ to repair it..

So I look at it..
Plug in - PSU LED came on, plug in monitor, disconnected all drives.. boot test.... "No Drives error... press enter" - so it boots..
I hit the Power button on the front, reconnect the drives.. and then it won't boot.. PSU LED off.
I recheck everything, unplug and replug everything.. PSU LED comes back on..
I boot it, I get stuck in the vista "I can't boot not even safe mode" boot loop...
I boot it into it's diagnostic tools and let it run the full set of tests under "I can't boot to the OS" ... Disk, memory, CPU all pass.
I go grab a Live CD, boot to it, force mount the drives, as they wouldn't mount right, seems ok at that level I can read the NTFS - I was missing the "ntfs" file from the CD to do the linux chkdsk..
I think hmm Vista is close enough to 7 I can get it to check it..
I get my Win 7 install CD - go to recovery console.
chkdsk /f it and well I get a pile of errors bad security descriptors etc..
I reboot it to the OS and I actually BOOTS, well I got a black screen with a cursor and not much else but it's a start right..
I then have to get to work so turn it off to work on it later...
That was all Friday / Saturday..
Today I go to boot it and nothing.
I then disconnect it from Power, wait, reconnect it and the PSU LED goes out..
I'm thinking "what" so i disconnect JUST the 20+4 main power conn..
The LED comes back on...
I hit the power button and well the fans spin for like 1/2 second and stop, the longer i press it the more it repeats it..

Thinking PSU or the mobo has gone ?
PSU is a 300w Hipro custom DELL job..

Thoughts.

Belated reply but never the less here it is, new PSU with a bit more Power to it.... and a reinstall of Win 7 and it works like a charm again...

Now to just go get me another laptop Hard drive as the last one is saying "Pre Failure" / "Old Age" in SMART when I use "Photo Rec" on the EBCD or it might be a UBCD I'm not sure used a few to test / check it.
Windows Hangs on Boot - eventually rebooting via a "Hard error" BSOD and then boots me to the recovery console...
In the recovery console - I can see the HDD no problem - checked it with CHKDSK /F - 1 bad sector!
It won't repair windows as it's a bad drive... :(
I'm hoping I can get an new 2.5" drive and clone over the old one ;) repair and get windows working, I intend to UP the HDD space so I got a Linux Space :) dual booter