Is there a way to simulate Access' After Update event in an Excel
spreadsheet? For example, if I had a column and someone entered a value into
it, can I write a routine to check the data and run other macros?

I realize that's not very clear and I know Excel has validation boxes, but
let's say I have a sheet that I want people to put an ID in the first column
of, the routine should check that the ID hasn't been previously used and
should populate the name, address... fields.

I'm not asking for anything that perfect to exist, but if someone could
either copy me a code chunk or point me to a good example online I would
really appreciate it.

TIA

Mark