I have the same problem and nothing iv'e tried has worked yet. Ok, so here's what happened. I've been using a router and my sister wanted to change to a hub since the router blocks file tranfers through messenger. When we switched to the hub my internet connection got really slow. So we switched back to our old router and my internet was still messed up. My internet is still slow and I keep getting disconnected from MSN messenger. My download speed always fluctuates from 0 - 60 kbps. When i try to repair the local area connection status it says error in flushing the arp cache. i tried using arp -a, arp -d in the command promp and still nothing happened. I tried disabling the "routing and remote access" from services and the reapair went fine but my internet connection still gets cut off. I tried using netsh diag gui and here's the results:

Internet Service
Default Outlook Express Mail Not Configured
Default Outlook Express News Not Configured
Internet Explorer Web Proxy Not Configured

Computer Information
Computer System YOU-U35A06OIXC9
Operating System Microsoft Windows XP Professional
Version 5.1.2600

Modems and Network Adapters
Modems Communications cable between two computers
Network Adapters [00000008] NVIDIA nForce MCP Networking Adapter FAILED
Network Clients

Any sort of help and advice would be greatly appreciated.