Installing the same Windows license on several partitions

[LoadWB]

tribz and mongo have it.  You don't need to worry about volume or OpenLicense keys, WGA, and all that crap if you do it correctly.  Even if you have to call Microsoft activation, you'll be fine.  Technically what you want to do is outside of licensing (depending upon who you ask at Microsoft,) but no biggie, just don't tell them that's what you're doing if you have to speak to someone.  Just act stupid like you're reinstalling multiple times and activate each time.  It works.

[LoadWB]

The thing is that at some point, I will have 2 copies of the same Windows at different partitions, both with WGA and switching between the two several times. I am not sure if this is allowed so WGA could start complaining.

It shouldn't.  Microsoft's policy for Windows 7 downgrade rights to XP is to install XP with a previously activated XP license key.  There's a maximum, but two partitions on the same computer activated once each should not trigger it.

[LoadWB]

No, they just don't care if people pirate Windows.  But in this case it's not even necessary.