Welcome to TSG.
I hope that you mean, Dell Dimension 9150, and not Dell inspiron 9150.
A lot of forums on Dells site mention this problem. 1 person seems have resolved the issue by powering off, unplugging the computer, and then holding the power button in for 30 secs to "drain" anything in the capacitors. Then hooking everything up and trying again. Others by trying every USB port looking for one that works.
Unfortunately it seems this is a common problem, and the most common solution is to replace the motherboard. You may want to remove the HDD, and hook it up as a secondary in another computer so you can get the files off.
For more details on the "unplugging & power button" directions can be found on page 2 of dells forum.
http://en.community.dell.com/forums/...px?PageIndex=2