I've set-up two separate user profiles on an old laptop running Windows-XP, and noted the following strange behaviour.
When the wireless network is setup using WEP both user profiles can connect to the internet and I can switch between user profiles, without network connectivity issues.
However when I changed the wireless security to WPA2, the first user to connect gets access to the internet, when switching to the second profile the connection fails.
The behaviour is the same regardless of which of the two profiles logs on first.
I also tried this with one user with admin rights and one without, and got the same result.

I'm not too concerned about a fix for this other than forcing the users to log-off when sharing the laptop, but I'd be interested to know if anyone can corroborate my findings.

The network config was done using Intel® PROSet/Wireless.