Please understand that all the ppl who help here volunteer their time to do so and cant be here 24/7. It may sometimes take a while to get a reply, and "bumping" your post wont get you help any faster. This problem has been addressed many times, using the search feature (set it to all forums and expand the date range) would have found lots of help.
First of all, if you are using xp, there is an option that causes xp to reboot on an error. Go to system properties (right click on My Computer on the desktop) click on the advanced tab, where you will find startup & recovery info, click on the settings button for it and in the new box that opens uncheck automatically reset. You will then be able to see the error that caused the reboot. Wouldnt surprise me if the error is 10053, software caused connection abort, in which case please read this thread
many things could be causing the problem in general, for example:
-your provider may be using some kind of interference technique to limit, or have, bandwidth limits
-you may be simply trying to do too much at the same time
-you could have a script/addon mucking things up, many ppl have reported similar probs using certain ones