When you open an Access database in a newer version of the software, Access will ask you if you want to convert it and then, if you reply 'yes' will do the conversion. When converting from Access 97 to Access 2000, it is recommended that you first compile all the code (see Debug-->Compile all Modules), then repair and compact it (see Tools-->Database Utilities). If you don't do this, you may get a message saying that the database is corrupt. This not does seem to be necessary when converting to XP.

If you reply 'No' to the conversion, then the database can be opened and data amended (I think), but no design changes can be made.

There is also information regarding conversions to/from different versions in Access Help. Use the keywords

convert, access

to find the relevant pages.