MS EXCEL Problem

Hello everyone,
Is there any Excel Master on this forum? I need some urgent help.
Here's my problem:
There are Two columns, the first represents product category (say products soap, brush, & comb), while the next column represents the serial nos. (say 1 to 10).
Each product can have serial nos 1 to 10, but once a combination is selected, it should not be repeated; for instance, if I select Soap+1 in a cell somewhere, the sheet should automatically reject the same entry elsewhere (i am going to use Data-Validation-List for this purpose).
The same should be the cas with Brush & Comb...
I tried using help, but to no avail.
Can you suggest a way in MS Excel in which it can be solved??
Your reply would be greatly appreciated.
regs,
Indiason

You will need to do a look up function that looks for repeated patterns.

http://www.techonthenet.com/excel/formulas/lookup.php

http://www.glencoe.com/ps/computered/pas/article.php4?articleId=411

Try this and see where you get. If you get an error, let me know.