I am trying to install an Intel Pentium III-S in an asus cuv4x mb

Do you have the Motherboard Manual for your Asus CUV4X? If not, here is the PDF version that you can download,
http://support.asus.com/download/download.aspx?SLanguage=en-us
On this page go up to the left corner. Under the Download heading, you will see -> Input Model to Search <- in a box
Put your mouse cursor in this box. The box will blank out. Left-click one time. Keep your mouse cursor in the box. Now type -> CUV4X

In the box underneath, where it states -> Select Category <-, left-click one time on the blue down arrow, to the right of it. Now click on -> Manuals.

On this page, click on -> CUV4X

Now click on your O/S (Operating System) I chose Win2K. Doesn't really matter, but I chose the newest Windows version they had at the time.
(Click on the blue down arrow, to the right of the -> Select an OS <- box)

On the next page if it's not opened up already, go down and click on the -> + sign next to -> Manual

Now go down to the heading -> Version E537 <-
in blue, and under this click on -> Global
(If something happens that it doesn't download using Global, you can click on China, instead. It's still in English)

You may not see anything happening for up to 30 seconds. Depends on the network traffic when you download. I downloaded it just now, and the first page popped up in 5 seconds. Took another 12 seconds to fully download. I use a medium speed DSL connection, and Adobe Reader 9.1
(Adobe Reader is on your computer. It uses PDF files)

WAIT, until the file has Fully downloaded, before looking through it. If you want to save a copy to your computer, so that you can look at it anytime you want, and also so that you can burn a copy off to CD disk, do this,

Go up to the top header, and left-click on the Floppy Disk icon. It is the second icon from the left. Hovering your mouse cursor over it, will briefly reveal, "Click to save this to your computer or another location"

After you click on it, a small window will come up.
Save a copy..
Unless you feel like looking for -> cuv4x-102.pdf <- on your computer, and on a saved CD disk, you may want to do this,

At the bottom of this small window is -> File name: , and a box to the right of it. cuv4x-102.pdf
will be in the box, and highlighted in blue.
Put your mouse cursor, right after the -> f <- in .pdf, and left-click twice.
Make one space with your space bar. Now type -> Asus CUV4X Motherboard Manual

Here's a list of Intel Pentium III Microprocessors,
http://en.wikipedia.org/wiki/List_of_Intel_Pentium_III_microprocessors

Why all this information? Because looking at the motherboard manual, it states 800MHz is the fastest Frequency Rate that it will use for a processor.
(Page 21)

The Intel Pentium III 1400S operates at 1400MHz, (1.4GHz), has 512KB of L2 cache, and a 133MHz
Front Side Bus. (FSB)

CUV4X can take an pIII 1.4GHz.
Check this:

http://biosupdate.runboard.com/t19

just you have to use a modified uP and upgrade BIOS