Cell Validation
Depending on how data ends up in the cells I'm not so sure that there is any need to use code (VBA) to perform validation.
If data ends up in a cell by linking or importing then maybe vba code is necessary but if you want to validate values / text etc as you enter then Excel's built in Data Validation should more than suffice.
There are some excellent examples of Data Validation at http://www.ozgrid.com/download/default.htm. Data validation when combined with Dynamic Named Ranges is extremely powerful and could replace vba code of reasonable complexity.
Alternatively you could do a search of the forum looking for postings on the subject....there are lots of postings.