The computer works fine when it is not connected to an internet connection. When you plug the modem in no matter if you use USB or intranet connection, the computer will just set there and continuously reboot. We've tried 3 different modems, all are good. We've tried new connector's, they all work fine on our 2nd computer, We've had the ISP come out and check the LINES everything checks out. This has to be some sort of internal problem with my computer. Has anyone ran into this problem before if so how did you fix it........HELP.....Need...more...info...gasp 

Operating system / antivirus & antispyware protection / service packs installed / when did it start happening / did it ever work right / installed any new software?

Alan <><  you might want to TRY to reintall your network card/modem drivers...or if all else fails replace the card/modemSystem is XP, AVG 7.5, Started happening about a week ago, computer worked fine up until then....no new software...Try doing a system RESTORE to a date prior to the problem starting / try samcarney's suggestion.

Alan <><  If these are PCI modems remove them and clean the contacts with rubbing alcohol and re-install them.
For the USB devices i would go to Control Panel/System/Device MANAGER and find every instance of USB roots and hubs...right click every one and select remove/uninstall. You will recieve a warning message...ignore it.
Re-boot the system and then re-plug the USB modem