i was up and running when a yahoo box popped up and said rebooting system for new operating system install...this is an old machine of mine that contains very important data that i dont want to lose and of course i never backed it up(moron)what do i need to do in order to get my operating system back without losing my data and files..i started a reinstall but was recomended to exit because it said detected o/s ...how can i install over the old o/s and not lose data or is this not possible and im gonna lose my data anyway....when i got to point to boot from hard drive i get the explorer error ..page fault and when i click to close it my pc just shuts down....safe mode doesnt work either and i have tried everything i know...please help if you can ...this is the first time i have ever saw this happen when i was up and running good....have some IT experience but obviously not enough to solve my problem...your help is greatly appreciated and thanks.

additional details:#1. I dont use this pc for adding anymore data...i just boot it up every now and then when my other one is busy with something else...Lshape desk with two pc's...and yes the last thing i saw was a box stating that yahoo was installing an application...i didnt catch anymore of it than that because it shut off to quickly...i currently have a year of virus and firewall protection on this pc as well...i dont care about it not being able to update widows..i already knew that..and when i rebooted my o/s was gone and asking me to install it...i thought i could just overwrite over the space where my o/s was originally...but getting conflicting answers to that question so i decided not to do anything until i got a little more input on that....no infections detected at all and everything was and has been running fine until that box popped up...it may be i accidently clicked something without realizing it (good poss.) and now i have a problem ...thanks.
i posted in hardware because of hard drive could be software related as well.

Well, I am an Soon to be IT. I would actually use Knoppix to get all your files and copy and paste them onto floppy's or burn them on a CD.