I had a recent loss of internet connection on all computers. AT&T sent their "repair guy" out, who switched my DSL input to a different external line, admitted that the line I'd been connected to was down, admitted that the line he hooked me to was "really noisy," but then stated that my house wiring is a bunch of crap and needs to be completely rewired. (They charge $150 minimum to look at anything inside the house, and something like $90/hr to actually do anything.)
He did, fairly willingly, offer some babbling about resets, that after translation I think I've reduced to something "intelligible." (?)
Both your router and your modem have two options.
OPTION 1: RESTART
You can RESTART either by just unplugging the power for long enough to let the internal memory clear, and then reconnect the power.
Instructions from different sources differ on whether the inputs/outputs should be connected when power is re-applied, or whether the device should be powered up and then inputs/outputs connected one at a time to let the device "find the inputs."
OPTION 2: RESET
Both modem and router should have a RESET button. On some, the reset may be done by pressing and holding a "power button." Neither of mine, modem or router, has a power switch, but both have separate reset buttons.
My AT&T guy insists that any time the MODEM is RESET, it is necessary to connect to the server and "log back in." The login may require both your account information and the "Modem Access Code" that should be shown on a label on the modem. The "login address" I've been told to use for the modem is 192.168.1.254, but I don't know if that's a universal address or is just for my AT&T service.
If any of the computers on your router connect, the modem probably doesn't need a RESET, and a RESTART should be tried first.
ADDITIONAL MAYBE:
Individual computers on your system may "remember" settings that, after a problem event may be incorrect. A couple of times AT&T haves recommended, on each computer, open a Command Window and run:
ipconfig/release
followed by:
ipconfig/renew
This allows the individual computer to "re-discover" the correct settings. If one computer on the LAN needs it, you probably should run the release/renew on each of them.
None of this seems applicable to the problem of "WinXP connects and Vista doesn't." It might help in more general cases of modem/router misbehavior.
My SWAG of the Vista problem is that it's coming from "security features" that Vista has and that other OSs don't, but I continue finding only "advertisements without technical content" at Microsoft, including at their previously useful Knowledge Base and at other "Support" places.