If you are lucky enough to have a 64 bit server and are trying to run packages that interface with access or excel you are probably having problems. This has been recently discussed on the MSDN Forums http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=167907&SiteID=1. The reason for the problems is that the Jet engine drivers are 32 bit and not 64 bit. The default 64bit version of dtexec tries to load the 64bit drivers and as there aren't any it fails. You therefore need to use the 32bit version of dtexec, this will try and load the 32 bits drivers.
The 32Bit dtexec can be found in the "program files (x86)" folder.