Depends on whether
1. whether yo have enough free space for windows
2. what Windows OS you are planning to install
If it is XP, you will need to boot into linux rescue mode to fix the MBR so it can boot from
lilo again. You would need to edit lilo.conf to make it aware of windows partition.
http://www.linuxquestions.org/questi...hreadid=198051
can guide you with this task of making lilo aware of the windows partition.