Troubleshooting painfully slow wireless connection

I was trying to help a friend with some issues he's having with his slow wireless connection and I don't really know where to look at this point. Here are some of the details around their setup and what's happening.

- He's using a plugin USB wireless adapter which is getting transfer speeds of about 0.2 MBPS
- When we drop an ethernet line from the wireless router to the computer we're getting transfer speeds of > 10 MBPS
- When we connect using the wireless card to someone else's wireless network the transfer speeds are approaching 10 MBPS
- He's using AVG software for anti-virus, firewall, internet security, etc. (disabling the individual software components doesn't seem to make any difference)

I'm confused because it seems as though if it were the USB wireless card it would be slow regardless of what network you connect to. Additionally, if it were the software (e.g. firewall) , I would expect it to be slow regardless of whether it's connecting via ethernet or wirelessly.

Any ideas would be appreciated.