need a little more info. as chev has stated is the connection wireless ?
Wireless isn't really the medium for transferring a great deal of files on a regular basis. It's fine for mobility and accessibilty but the cost to you would be contention and lowered throughput.
if you aren't already I would suggest using a dedicated CAT5 LAN cable.
however, if you are using UTP, I have resolved quite a number of these sort of issues (here and in my place of work) with a simple swap of the CAT5 cable. There are a couple of reasons for this; firstly slow throughput can occur if the cable is damaged, also if it's not correctly rated for the transfer speed that you have requested.
other questions, have you any AV within your network ? is it scanning on reads and writes ?
what sort of transfers are you doing, lots of small files or a few large files?
what are the differences apart from OS between you XP machine and the 2 Windows 7, hardware, transfer software, AV, Firewall, etc ?