The Satellite \ Satellite Pro P300 \ P300D Series suffers from Problems with Wireless Connectivity.
To solve these problems you need to upgrade to at least Bios Version 3.00 (recent is 3.10).
For XP and Vista64, that should already fix it.
For Vista32, you need to upgrade the Wireless Lan Drivers as well.
But I would recommend upgrading it also on x64 Systems.
(disabling autotuning for networking and installing sp2 should help as well!)