Yes you can, and since you are performing a clean install on the same machine to which that key belongs then the MS EULA explicitly allows you to do so. MS only complains if you are trying to move it from one machine to another.
While for most versions of windows you need to use an OEM disk, with windows 7 MS actually had a flash of non stupidity and this is not required. Just install and when you put in the key it will automatically switch between retail and OEM version of the OS as appropriate. You do need to have the right version in terms of pro/home/enterprise etc though.
Interestingly for certain versions of windows the preinstalled version doesn't even use your key (the one on the sticker). instead using a bulk key specifically for OEMs like toshiba to bulk activate installs with. Where the key on the sticker is specifically only to be used when the customer performs a clean install on their own.
You should be able to get the installation disks you need from Toshiba themselves. Or from microsoft if you have an MSDN subscription.
Yes, maybe, yes.
Horrible answer I know but it really depends on the versions. Most of the time if you have an OEM CD then you'll be able to use the code. However, if you have, say a Dell install disc and you try and use it, it will not accept another computer, since it checks.
Worst case, you'll need to find the version that will allow you to install with that specific key.
