Hi, I have a weird setup of:

[USBModem]---------[CompA] > > > Wireless > > >[CompB]------Wired-------[CompC]

But whenever compB is connected to compC compB cannot access the internet. All computers are running Windows xp Pro. Any ideas how I let CompB and CompC access the internet at the same time?

Bridging on CompB doesn't work nor does re-ordering the network adapters.

Thanks in advance

There's one thing I can say here and its YUKK!!
What a convoluted way of sharing your connection.

You need a Router as USB isn't an efficient way of using a Broadband connection.
Now the exact type you need depends on your internet connection, either Cable or DSL.
Here's a coupld of How-To's for networking
Wired networking Guide
Wireless Networking Guide
Should give you some more ideas.

Remember the KISS Principle.

I agree it isn't ideal but it's only temporarily and being a student, buying an ADSL Router is out of the question really.

Really? Have you shopped around? You can get broadband routers for as little as $20, even less if you catch a good sale.

Not ADSL ones in the UK, everything is expensive over here otherwise I would have already got one, also I haven't got any free slots for a Wireless card (either PCI or USB) in CompC.

Gameon, so you are in the UK and a student.
Try Ebay !
Broadband routers have normal network ports as well as wireles access.
Let me know what kind of budget you have and I'll find you something suitable.