Hard disk boot sector invalid
When you get this error when you boot your computer, you know something is terribly wrong.
Actually, in most cases it’s not. What probably happened was that you did not have set a bootable partition on your booting hard disk.
How can one not set a partition as bootable? It can happen when you install a fresh Linux distribution using the manual partitioning option, and you shamelessly forgot to toggle the bootable flag on your Linux partition. However, this implies that you already got rid of WXP, so you are totally excused.
Another reason for the bootable flag missing is that you have erased the said partition and recreated it.
How do you set the bootable flag again?
You can boot with a installation CD/rescue CD and set it using the partitioning tool. There is no need to install again.
Since you already have Linux installed, you can boot with the Ubuntu installation CD and choose the last option, Boot from first hard disk, to boot of Linux. Then, use the distribution partitioning tool to set the bootable flag.
Update 23Feb07: The bootable flag has to be set on one of the primary partitions. It does not work if you set the bootable flag on a logical partition. Apparently the above error message comes from the BIOS, which blindly does an unneeded check to see if a primary partition (whatever primary partition) has the bootable flag set. The primary partition you set the flag on does not have to be a partition that you can boot from. Any primary partition will suffice. Once the BIOS relinquishes control to the MBR, GRUB takes over and brings you to your Linux distribution.