Odd thing with v7:
In Powerpoint 2003, Vista 32 bit, just using 'New' document (blank) the first tab is Doment1, but hit New again and Document2 tab overlays it. Hmmm.
Next is Document3, but closing those and- hey, there's no Document1.
This doesn't happen in Word or Excel.
Also if you manually close e.g. Document 3,2,1, so you have no documents open, then open another one, it's Document4.. I'd have expected (maybe it's just me) Document1. Being more extreme, if you had 13 tabs, closed them all and opened another, it becomes Document14..