I assume that you read the following, http://support.microsoft.com/default.aspx?...p;Product=winxp
Before I ever consider a BIOS update (haven't done one since Windows 98), I would replace the CMOS battery (less than $5 item) on the system. The reason I would do this is that the CMOS battery is a known element which, when failing, can make the BIOS/system behave strangely.
I'm curious as to what manufacturer and model apply to this system.
<<For some time now...>>
How long is that? Has this system ever run smoothly? Why have you ignored this until now?