If you want to run the M1015 natively then it most closely matches the MegaRaid 9240-8i. You are right - there are no MegaRaid Win-XP x86 drivers on LSIs site (kinda weird...), but there are Server2003 x86 drivers. The driver model is identical between XP and Server203 so you could give loading those a try. I've done this in the past for other hardware with good results. YMMV.
Alternatively, you could re-flash the card using SAS-9211-8i firmware. While there are no MegaRaid drivers for XP x86 on the LSI site, there are SAS-series drivers for XP x86. You'd lose any Raid5 capability - but I'm guessing you don't have the IBM raidkey anyway so that is probably a minimal problem.