If you are like me and simply didn't think about rearm windows again in
time, you're faced with a problem. You can only access you computer
after that in reduced mode. Starting your computer up in
safemode+commandprompt doesn't help either, cause this mode doesn't give enough
permissions to run the rearm.vbs script.

What you is as follows:

(in case you have firefox as default browser)
1: Start your pc up safemode+commandprompt.
2: type explorer in the commandprompt window.
3: startup internet explorer and set it as default browser again.
4: reboot.
1:start up vista normally
2:choose reduced mode to buy a key online
3:Internet explorer will open, type "c:" without the quotes in the
4:Screen pops up, type "cmd" in the adressbar.
5:Navigate to windows/system32 directory and execute slmgr.vbs -rearm,
or in case you used the hacked version, slmgrhack.vbs -rearm.
6:Wait a few seconds, a command successfull screen will follow.
7:Reboot, windows will tell you have 30 days left to install.
8:Presto, you are back in Vista.

Interesting tip.