If you have original retail copy of XP then you should not have a problem with reinstallation of XP. You may need to contact MS by phone if your registration is rejected but that is not a problem. If you only have an OEM copy then you may run into problems both physically and legally but since you didn't have a problem with the new hard drive installation then I assume your XP disk is the retail version. I don't understand what you mean about BIOS problems. I assume that your planned CPU is designed to work in your MOBO and you can easily check that out at the ASUS site. Also make sure that your memory is able to work in the MOBO at optimum levels. I hope this helps but if not I'm sure there will be other comments along soon.