I haven't had to do that in a while but I used to use a 98 boot disk and use FDISK /MBR. Here's link to MS page
http://support.microsoft.com/kb/69013/

You can restore the MBR by booting from your OEM Windows XP CD and entering the Recovery Console which is fairly simple. All you need to do is type fixmbr (you'll need to type fixmbr C: or whatever the boot drive is if you have multiple HDDs) and type yes/y when prompted, and you should recieve a message showing it was successful. Then all you do is take out the cd, reboot and everything should be fine.

You WILL need to know the admin password in order to enter the recovery console.

HTH

(once you get windows running again you can install the recovery console as a startup option by inserting the windows cd-rom and clicking start>run and typing "d:\i386\winnt32.exe /cmdcons" where "d" is your cd-rom drive letter. I've been trying out a lot of different linux distributions and it is much faster to restore the mbr to original now that I can boot straight into the recovery console w/o going through the win pre-install process)

I know this is an old post, but I have a Inspiron 5150 and would like to get a little more use out of it. would like to know why you did not keep ubuntu on your laptop. I was going to install it rather than throw this computer in the trash. It has been the worst computer I have every owned. I was concerned that this free linux program might not be able to use the disc drive or the power step might operate at full fan. Would appreciate your experience. Inspiron laptop still has the original Phonix bios