I was using RedHat 5.1
Then I changed my mind to use Slackware. So I downloaded Slackware 12.2
I formatted my harddisk and partitioned it in three parts.
1 > boot
2 > swap
3 >/

I installed slackware in /
After some hour, the installation was complete.

It told me to exit setup and restart by ctrl+alt+del
I did it but when my system reboots it says:
Error loading Operating system..

What is wrong in this case?
How to solve this problem?

Did you make the boot partition bootable? Did you install a bootloader such as Lilo or Grub?

I'm sure that I did not install Lilo but I am confused about Grub.
Can any one give any information how to install Slackware

Lilo and Grub are both bootloaders and you need to have a bootloader to boot the system. As for help installing Slackware, the best source is their install docs - http://www.slackware.com/install/.

Most of that documentation doesn't apply any more (well, sections 1-3, anyway). All you should need to do to install is partition with (c)fdisk and then run "setup". The rest should be self-explanatory. errorsunknownp, as hallamigo stated, you'll need to install a bootloader. When you install Slackware, you'll be asked whether you want to install LILO and if you choose not to, you can still boot your installation using CD 1 (or the DVD). If you boot from the CD/DVD, you'll see instructions for doing this. You can install GRUB if you wish, instead of LILO, but you'll need to do it manually, since Slackware's setup program doesn't let you choose between LILO and GRUB.

I installed LILO
After that I didn't find message - Error loading operating system

But it shows that L 99 99 99 99 99 99 99 99 99 99 99 99 99

then it stucks

I waited it for more than 5 min
nothing happens

What is happening?

This is a common error message, so you may want to see the other threads on it here (or other places on the web, of course). Can you boot your installation from the CD/DVD? If so, it'd be good if you could post the contents of /etc/lilo.conf here and perhaps the output from "fdisk -l" (that's a lowercase L, not number one).

1. make the /boot partition bootable( a primary partition )
2. install slackware in /
3. install lilo in /boot, not in mbr
4. if you make two partitions active(i.e. Windows C: and /boot in same HDD), no OS will boot

or you can simply make an swap and and a / partition and install bootloader to MBR when prompted