Hey there,

I have a computer with the 2 following operating systems installed on the same hard drive: Windows XP and Red Hat 9. The boot manager I used is LILO. I know I just have to erase the linux partitions to get rid of linux on the system but if I do this I also get rid of the boot loader. How can I simply put my computer back to the way it was before I installed linux, when there was only win XP installed on it.

To erase linux, just format the partition with ntfs or fat32. You can use xp's own tools for that. To uninstall grub or lilo from the bootsector, use winxp console recovery:
Boot the computer using the xp install cd. Then, choose recovery console, instead of new installation. Log into the installation. You'll be prompted for the admin passwd.
Once there, you can run the xp utility to write a boot sector, called fixboot, or fixmbr, something like this. Just run the tool, it'll write the new boot and overwrite grub or lilo.

Post again if you're still in doubt.

just erase the linux partition from win then from a startup disk when ur in dos type
fdisk /mbr

or from the winxp cd in the recovery console type
/fixmbr