Very poor download speed on XP, very GOOD speed on Ubuntu, same hardware

[#1]

Fibre optic was recently installed, but my ethernet connected desktop (XP, SP2) could not achieve more than about 5Mbps. Laptops connected into the same cable could achieve well over 30 Mbps.
Thorough hardware testing of the desktop revealed no faults. So I built an Ubuntu boot CD, and ran that up. On the same hardware as before, I immediately achieved 36 Mbps.
It seems that the XP software is choking the download performance by a very huge amount. Has anyone come across this before? Even more to the point - has anyone got any suggestions as to how I solve the problem (apart, that is, from junking Windoze and going to Linux!)?
Peter.

[#2]

This thread might be worth having a look at https://community.plus.net/forum/index.php?topic=102910.0

[#3]

Have you tried XP in Safe Mode with Networking?  This won't help at all if dvorak's suggestion is correct but would point to a software issue if you see an improvement.

[#4]

Make sure that the MTU setting on the router is 1500 - it could be as simple as that.

[#5]

MTU and RWIN. 
My moneys on RWIN, it probably needs to be increased a LOT. DrTCP is a useful tool for adjusting it.

[#6]

Thanks very much for those helpful-looking pointers. I shall find some time to work through them this weekend, and will report back.
Peter

[#7]

I think the default RWIN in XP is about 17000. I'd increase it to about 100000 on a 30mbps line.
IIRC on my 20mbps connection I set RWIN to about 60000 on XP systems that need it doing - not all it depends on the adapter in use and its drivers for reasons I haven't bothered to investigate.

[#8]

LOL - I'd forgotten I'd written that post. But it certainly made a difference.
However, I discovered something else when I first had fibre installed 6 months earlier, that restricted my speedtest results even more dramatically.
That was that some plugins on Firefox made a *huge* difference to the speed that the test ran at. IIRC I was limited to something less than 20. In that case, it was the firebug plugin, used for web-development activities.