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.