I had to reinstall XP on a dual boot with Slackware 8.1 and now lilo is gone. It just boots directly into XP. Is it possible to rewrite lilo to mbr again?

boot to linux from CD/floppy and run "lilo" as root

yes... Use your instalation media (CD) to boot from it again
and at the boot prompt enter: root = /dev/hda1 (or whateve your partition is installed on) and after that enter the setup program and go to configure and follow the instructions

You could also use the bootloader for xp to boot into your linux system.
write the lilo on the / drive instead of MBR.

Then use a program called bootpart in xp to create the file needed for the bootloader to boot your / partition .

In that way you don't have to mess with the MBR every time you reinstall XP.

Since you have a new XP install, you may want to do that over again after repartioning your drive with a small Dos or dos32 partition (hda1) before the XP's (ntsc) partition. This would allow you to use lilo instead of the ndldr. If the first two partitions are primary, linux will still start in the hda5 partition. I have an HP laptop with XP preinstalled, and it does this, so I didn't have to mess with boot.ini entries when I installed linux.

This is an example using the Windows "ntldr" to boot XP,Debian,Slackware. I created a softlink from the /boot/vmlinuz-ide-2.4.18(vmlinuz or bzimage) to "/" in order to forego the pleasure of having 2 bootscreens and booting straight to the kernel. The examples are from Slackware but they're generic. Here's an example of lilo.conf with everything commented out except the essentials.

# LILO configuration file
# generated by 'liloconfig'
#
# Start LILO global section
append="hdc=ide-scsi"
boot = /dev/hdb6
#message = /boot/boot_message.txt
#prompt
#timeout = 1
# Override dangerous defaults that rewrite the partition table:
change-rules
reset
# Normal VGA console
vga = normal
# End LILO global section
# DOS bootable partition config begins

# DOS bootable partition config ends
# Linux bootable partition config begins
image = /vmlinuz-ide-2.4.18
root = /dev/hdb6
label = Slackware
read-only
# Linux bootable partition config ends

Note that the image= is not in the /boot. Run "/sbin/lilo" If you get a "Warning" that this is not the first partition. You did good. Now create your bootsect file.

root@whatever#dd if=/dev/hdb6 of=/bootsect.slk bs=512 count=1

Copy the bootsect.slk to the drive where "ntldr" and "boot.ini" reside. (Normally C)by using a dos format floppy or e-mail it to yourself.If your Windows drive is vfat just mount the drive and copy straight to it. Then add a line to "boot.ini" Here's mine.

[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect
C:\bootsect.dbn ="Debian"
C:\bootsect.slk="Slackware"

Reboot and pic your flavor.