Yes, you can; the game has been succesfully tested to run on Windows Vista.
The game automatically scales to your monitor's resolution when in fullscreen mode. You can press F4 ingame to switch to non-fullscreen mode.
You will need at least 9 MB of free disk space to install and run the game. It does not run on any operating system apart from Windows unless you use an emulator. The rest of the requirements are not really known to me but I suspect any computer not older than 6 years should be able to run it successfully. And since the game download is only 6 MB it wouldn't be too much of a hassle to just try if it runs. (Let me know though if you run into serious problems).
Well the current version contains:
Yes there are! There are two types of champion monsters actually. The first one is just a stronger version of the same monster. The monster basically earns a golden aura, a weird sounding name, and some improved statistics (these include additional spells). They are quite common. The second type are unique monsters (kind off like the superunique monsters in Diablo II) which have a given name and abilities but do not always appear. They often have minions, stronger versions of normal monsters, with them. Killed player characters may also return to haunt future adventurers as a unique monster.
Summoned creatures and allies which may join you can be given basic commands such as where to go, who to attack, which spell to cast, etc. You can also change their general behaviour or unsummon them. The game features a Tactics screen in which the game is pauzed and you can give out commands thoughtfully.
The game does include the permanent death option (in fact it is enabled by default). If enabled your character will only have a single life. Should you die your savegames will be automatically deleted, but your corpse may turn up in future games.
No, the game automatically saves for you everytime you enter a new dungeon level (which happens usually every 7-10 minutes).