I recently changed my isp and was provided with a usb modem, this opened with a speed of 2.6 meg regularly. I then reprogrammed my router (d-link dsl g-604t) connected by cable to 4 computers, this equally regularly gives 100.0Mbps. Why the difference and can I speed the router up

Sorry to say, but you are getting slightly confused.
The USB modem is reporting closer to what your connection speed really is.
The 100 Mbps from the router is correct as far as the connection ton the router ONLY is concerned. The routers connection to your ISP is different, but you can get a good idea what it is if you have a look at the sync speed.
Just log on to your router and you'll fine a line similar to this:


Bandwidth (Up/Down) [kbps/kbps]: 1,183 / 14,947

This isn't your surfing speed though as there is some overhead to take into account.
There are a number os speed test sites available for you to check your connection.
Just try a quick Google ;)

Ps, stick with your router and dump the USB as the router offers better security and powerful networking.

Thanks , thought there might be a simple answer and I am sticking to the router, which is why I asked the question