How to lock a table while a transaction runs

I'm using Con.BeginTrans etc. so that i can wrap into one transaction a query that gets the MAX value of an id field out of my database and a query that then uses this with some other data to do an INSERT back into the database. Can anyone tell me how i can lock the whole table while the transaction runs to eliminate the possibility of two people getting the same MAX value for the id. This is the only way i could think of doing it.

Thanks in advance
Ian.