|Re you letting xp handle wifi connection, or are you using the Intel driver/utility! This of corse presumes your wifi adapter is an Intel based one.|
I have found with my very elderly, olde, Acer Aspire that its wifi is much happier and more stable using the Intel divers (sourced from Intel support/downloads) than anything in xp, or from Acer.
Also is the laptop on mains or battery power when the shut down of wifi occurs? If battery then it may be conserving battery power?