I want to check my Fs with reiserfsck and here what he writes:
[root@ ~]# reiserfsck /dev/hda1

<-------------reiserfsck, 2002------------->
reiserfsprogs 3.x.1b

Will read-only check consistency of the filesystem on /dev/hda1
Will put log info to 'stdout'

Do you want to run this program?[N/Yes] (note need to type Yes):Yes
###########
reiserfsck --check started at Thu Nov 21 19:32:59 2002
###########
Device /dev/hda1 is mounted w/ write permissions, can not check it
[root@ ~]#

Any ideas are appreciated

It is complaining because the file system is currently mounted and it is probably your root file system. I would suggest the following to accomplish what you desire.

1) Boot your Slack 8.1 install CD and login as root.
2) mount your file system as: mount -t reiserfs /dev/hda1 /mnt
3) copy reiserfsck: cp /mnt/sbin/reiserfsck /sbin
4) unmount the file system: umount /dev/hda1
5) run reiserfsck: reiserfsck /dev/hda1

I would also suggest you read the man page for reiserfsck prior to attempting. It has many options that you may want to use in the event there are errors reported.

Good luck.