I have Windows 7 installed along with Slackware. Now I wish to remove Slackware and LILO and try CentOS.
The problem is that I installed LILO and Slackware a long time back and now I don't remember where I installed LILO.
I believe deleting the partition of Slackware will remove it completely. Am I correct?
How do I remove LILO after that?

Is LILO the boot loader for both windows and Slackware ?
In that case LILO is installed at MBR of the disk .
If LILO is installed at MBR removing the Slackware partition than LILO is still in MBR .
If you installed Centos boot loader in the MBR than it will overwrite LILO .
IF LILO is installed at MBR you can also use the repair option of windows 7 cd to overwrite LILO with the command bootrec.exe fixmbr
Or did you use two boot loaders ?

The major portion of Lilo is installed in the root partition. But, a small part is in the MBR (Master Boot Record) portion of the hard drive, along with the partition table.

If you delete the Slackware partition, that will not remove the part of Lilo which is in the MBR, and you will not be able to boot windows.

You can install CentOS, and during the installation process, choose to install in the partition which now contains Slaceware. Choose to reformat the partition during installation.

When you get to the part of the installation which asks where to install Grub, choose to install to the MBR of the hard drive. That will overwrite the Lilo part in the MBR with the first stage of Grub.

The installation of grub should detect the windows installation and make a generic entry for windows into the grub menu (it will not name windows 7; it will only say windows in a partition number).

Finally did it. It was as "bigrigdriver" said. I deleted the partition having Slackware but LILO leftouts were the problem.
I had seen many solutions but they all were related to old windows version where fdisk still worked. But with windows 7, this is not the case.
I was keeping Windows 7 installation disk as the last resort because I had to create a USB stick with windows installation which is a pain but hell, had to get this over with.

Thanks "ronlau9" and "bigrigdriver" for your help.

This is what I did to get through this:
1) Boot with windows installation disk.
2) Click on repair and then select command prompt.
3) Change the directory to USB and then to boot folder.
4) Execute this: bootsect /nt60 c: /mbr
5) Restart. Done!