When I recently installed Ubuntu 11.10 on my desktop, it took a long time too because I had selected to download and install updates during initial install. For what it's worth, my install was 3+ hours long and I have a fairly quick cablemodem connection.
Should I reboot the computer?
No, just let it sit for a while and it should come back.
If you're interested, you can use the CTRL-ALT-F1 to get to a text console and tail the install log to see if it is stuck at some step.
If it does get powered off, you should be able to re-start the installation and it should be fine. If you do restart, don't select the updates option - it should go much faster since it will only use the DVD install media files.