First of all, if it came with 64bit you were already licensed for 32 bit. You should not have bought it.
Secondly, there is almost no reason. Not to use 64 bit, and if you have 4GB of memory you ABSOLUTELY want 64bit or you won't be able to use all your memory. 32Bit should only ever be used if a specific piece of critical software is incompatible.
thirdly, why are you asking about XP when you said you installed 7?