The earliest version I ever used was Access 2.0 - I don't know anybody still running it, but I'm sure they are out there.
Then came Access97, Access2000, Access2002, Access2003 and now Access2007.
From Access97 and Access2003 there was little change to the core features, just peripheral changes. 99% of what I do in Access2003 I could have done in Access97.
Access2000 introduced an MDB format that was incompatible with Access97's. In 2000, you could read the 97 data, but not modify it. You had the option of converting the database to a 2000 format.
2002/2003 offered another new format, however you only needed it if you wanted to use some of the new "features" of that version. Curiously, the default format for a new database is 2000 in 2002/2003, you have to tell it specifically to use the 2002/2003 format if you want it. I always use the 2000 format, so that the database is compatible with users running Access2000. Plus, there is talk that the newer format runs slower.
I can tell you little about Access2007 as I haven't used it yet. From what I've read, it has made quite a few changes in both the core and peripheral features. The jury seems to be out so far on whether the majority of the new features are improvements. And the file format is again completely changed and incompatible with earlier versions.
I'm not sure where you could purchase earlier versions, possibly EBay. My understanding is that having a license for a newer version of Access automatically gives you a license for previous versions (but don't take my word for it).
