By coincidence I found that a 64 bit OS is using an E1000 network adapter. This was chosen as default by ESX. So I checked a couple of servers and for my 32 bits servers it took the flexible adapter which use the VMXNET dll in Windows. I guess these settings are inherited migrations from ESX 2.5 to ESX 3.0 and now ESX 3.5 update 3.
Now there is also a VMXNET 2 (Enhanced) adapter available from ESX 3.5 that I want to use. More information you can find here: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1001805
I'm excited and test two VM servers. One I can change and by the other I see only the option flexible. So no VMXNET 2 (Enhanced). Both OS'es are equal. Windows 2003 Enterprise R2 SP2 32 bit. Both servers rolled out from the same template. So what to do.