Doesn't matter where you begin at the command prompt, vssadmin would run under c:\dogpoop\idontlikeit for all it cares. lol
To give you piece of mind though, type in CD\ at the command prompt and that will start you off right at the root C:\ ..
If you need help using the switches with vssadmin, you can google it to get really in depth information about it. If you don't feel comfy with that, reply here and I'll personally walk you through vssadmin commands if you so desire.
MCDST:XP, MCTS: Vista, MCITP:EST: Vista