I solved the problem. I already head the latest ODBC drivers installed, but they didn't work. I also searched the internet for a while till I found this webpage.

http://www.visualpractice.com/xp_merge.shtml

There is the answer for the problem with WinXP and the Mailmerge problem I had. You must reinstall the odbc-driver and you must run a tool named "Visual FoxPro DSN Enabler". Only reinstalling the ODBC driver isn't enough. They can both be downloaded from this website.

Greets,

Sander