Pillow,raraken,Ken and something i forget.
Walao, now you drag me in 
The above problem is very vague. How does the desktop connect to
the router? What router is it? (Brand, model etc.) OS in
question?
I kena this before, my desktop connected via Ethernet, my laptop
of course wireless. Then got alot of other computers in the house
that aren't mine. Every now and then this will happen to one of the
computers, then have to diagnose, solve, yadda yadda yadda.Time
consuming, and pisses me off.
What you can try: Go to Internet Explorer (IE)
Assuming you have the latest version of IE, go under Tools
Internet Options, followed by the Connections tab. At the bottom
you should see the 'Lan Settings' button. Click it, and then check
the box 'Automatically detect LAN settings' or something. See if it
works. Alternatively, you could try to have your router assign a
new address to your computer.
If both fail, then you might need access the router straight via
its IP. Its a last resort, sa the wrong move can disable the
internet.