Help. My Belkin wireless adaptor fitted in USB port has melted and taken my pc with it. Bad burning smell, adaptor totally trashed and now Nothing works. Anyone any similar experiences or ideas?

You have voided your warranty by letting the factory pre-installed smoke out of your system.

But seriously, to answer your questions, yes lots of us have had a system meltdown.
As to the actual cause only a technician with hands on might be able to work out which of the components failed first to cause the problem.
It doesn't really matter. That "Nothing works" now is indication enough that your meltdown could involve any/all of powersupply, motherboard, CPU,ram,graphics card, other PCI devices, drives etc.
You won't know until you can test each component in a known good compatible system. Your assumption that the wireless USB caused the problem may or may not be correct. it is just as likely that some other failure occurred within the motherboard that resulted in overcurrent being passed through the USB port.

I used to have a signature but it disappeared and I just couldn't be bothered writing another so please feel free to ingore this.

"Your assumption that the wireless USB caused the problem may or may not be correct."

The USB ports are supposed to be supplied with an accurate 5 volts and up to 500ma of current - if the USB device had shorted, it would have zapped the circuits connecting it to the mboard, but it's very unlikely the mboard would be dead after that even if it only partially worked. What's a lot more likely is the PS was failing and produced voltages that were too high, or the PS shorted. It is very common for a PS to fail, and sometimes it damages a lot of things attached to the mboard when it does.
Check out your PS -
See response 4 in this:
http://www.computing.net/hardware/w...

If it was the PS that failed, your cpu may be dead as well as your mboard. The ram and hard drive(s) usually survive intact but can also be damaged; CD drives, floppy drives, and sometimes cards that were in the slots are often damaged as well.

I agree with the above. It's probably the power supply or motherboard. Very likely to be both.

Do yourself a favor BACKUP!