I think you need to press "Ctrl + F11 or it maybe F7" during boot and it should give you a menu. Also, if you check your receipt, it will mention if you should have a copy of the OS. If so, call Tech Support in Ireland and tell them you would like them to send you the missing OS, plus the driver disks. If it doesn't mention anything about the OS disk and you are within the return period ie: 7 days, tell them its an incomplete system and you would like to return it. Otherwise, just ask Tech Support for the disks and they will in all likelihood give them to you.
Also, when you first booted your system, it should have given you the option to burn a copy of the disk. If it didn't report this to the Tech Support Department and they will probably send you a copy of the disk. If not, then use something like this http://www.petri.co.il/quickly_retri...ows_cd_key.htm
to retrieve the key, then get hold of a copy of the OS disk doesn't matter from where, it can be borrowed from a friend or whatever but it needs to be the same flavour ie: Home or Proffessional and reinstall. Any problems, call Microsoft and tell them you recently replaced (NOT upgraded) your motherboard due to it crashing\fusing on you and they will give you a replacement key in exchange for the number on the tag at the bottom of the computer. If they don't, there is an automated system in place that will give you a replacement key.