Tried installing Scientific Linux, but my pentium M of 400Mhz doesn't support pae. Wondered if SL has any unmentioned options for using a different kernel like slackware provides? If not does Centos have such a non-pae option?

Upstream (that means Redhat) have made it a requirement for release 6 - so both SL and CentOS will be impacted (and going forward of course).

If your Pentium M is not a "Banias" - see list here - it's possible it is one of the models that incorrectly don't report the PAE flag when in fact they are PAE-capable.
The kernel accepts a parameter of "forcepae" to accomodate this. Ubuntu have updated their installer to accept (and honour) this if appended to the kernel command line at install time. I don't know if that will work with the RHEL variants, but I'd be willing to bet it doesn't. No harm in trying though if you have a non-Banias chip.