By default, Compound TCP is disabled on Vista, but enabled on Longhorn server, unless this changes before Longhorn server ships.
CTCP is a technology that came out of the Microsoft Research group as a way to deal with connections that have a large
bandwidth-delay product.
A good example of this kind of connection would be satellite internet (DirectPC / Starband) or wireless technology like EVDO or WiMax.
To enable CTCP on Vista, run 'netsh interface tcp set global congestionprovider=ctcp'.
To disable CTCP on Vista, run 'netsh interface tcp set global congestionprovider=none'.
If you are going to use CTCP on a lossy connection that might have packets come back out of order, you might also want to enable RFC 1323 timestamps, unless you are on a very low bandwidth connection. You can do that by running 'netsh interface tcp set global timestamps=enabled'.
For a list of the global TCP settings under Vista (and Longhorn), launch a command prompt and then run 'netsh interface tcp show global'.
You could almost think of the tunable settings in 'netsh interface tcp' much like the tunable settings under /proc/net/ipv4 in Linux.
A good writeup on CTCP can be found
here.
Information about the new TCP/IP stack in Vista/Longhorn can be found
here.
For a real world example, with CTCP enabled, I can saturate a cellphone data connection (1xRTT or EVDO) and have smoother performance overall. I suspect I'd notice a bigger change if the connection was EVDO Rev A compared to EVDO Rev 0.