An MTU of 1500 is "normal" for ethernet. Mind you, windows uses Path MTU discovery to set the MTU.
http://www.netheaven.com/pmtu.html
http://en.wikipedia.org/wiki/Maximum_transmission_unitUnfortunately, increasing numbers of networks block all ICMP traffic (e.g. to prevent denial-of-service attacks), which prevents path MTU discovery from working. One often detects such blocking in the cases where a connection works for low-volume data but hangs as soon as a host sends a large block of data at a time