With windows (WZC) controlling the wireless, in Network Connections right-click the wireless card and choose Properties. In the Properties window that opens, go to the Wireless Networks tab. Then click the Advanced button in the lower right corner of the dialog. Tick the 'Access point (infrastructure) networks only' radio button, and ensure the box next to 'automatically connect to non-preferred networks' is UNchecked. See attached.
It could be seeing an ad-hoc network advertised by the other laptop and trying to connect to that (ad-hoc network) automatically (even if it's not a listed, or 'preferred' network), then when the other laptop connects to the infrastructure network it stops advertising its ad-hoc network(s) so your toshiba connects to the AP again.
If that fixes it, look for similar settings in the toshiba connection manager.
Prior to SP3, the default settings on XP were 'any available network' and 'connect automatically to non-preferred'. If you had the toshiba connection manager in control of the wireless when SP3 was applied, that security hole might have been missed in WZC, and SP3's setup would not have fixed similar security holes in 3rd-party connection managers.
