Desktop(slow) vs laptop(fast) internet speeds

In the past month or so, I have noticed my desktop pc internet download speeds have slowed down. I contacted cable (comcast) and they said I am on 6 meg line. I'm getting about 500k. cable guy came out, checked, everything OK.

I then tried connecting my laptop to exact same ethernet cable from the back of my desktop. Good speed.

Cannot figure out why desktop is slow, but not laptop.

Details: (using Speakeasy.net to Chicago (closest to my home))
Desktop:
Download Speed: 3890 kbps (486.3 KB/sec transfer rate)
Upload Speed: 2663 kbps (332.9 KB/sec transfer rate)
Laptop:
Download Speed: 21627 kbps (2703.4 KB/sec transfer rate)
Upload Speed: 2674 kbps (334.3 KB/sec transfer rate)

Desktop XP Home SP3, 512mb ram, P4 2.4 ghz
AVG 8, COMODO firewall
Laptop XP Media Center SP3, Intel Pentium M, 1.63ghz, 504 mb ram
AVG 8, Windows Firewall

I had Zonealarm and replaced it with COMODO, Avast and replaced with AVG - no luck

One more thing - when I open windows firewall, the ON button is greyed out and I cannot turn it off. There is a message that says "some setting are controlled by Group Policy". OK - How do turn Group Policy off?

I searched thru Majo Geeks and other sites trying to find an answer and have no luck so far.

Thanks in advance for your help.