How to lock a cell after making entry in Microsoft Excel

[SirName]

I am creating a test using excel 2003 that will self upgrade and give the tester information to review when they get the answer incorrect. What I would like to do is only give them one chance to answer. I do not want them to be able to change their answer. I would like them to enter their answer in a cell. If they get it incorrect, through data validation, a box will pop up to tell them what they need to do, by going back and studying. When they close the data validation box, I would like the answer to stay incorrect and the cell to lock so that they can not change their answer. Can someone help me with this.

[Essien]

I would suggest you re-think the Data Validation message part. If they get it incorrect, data Validation will pop up a message telling them they have it wrong and "retry" or "cancel". Cancelling will leave the cell blank. You can lock a cell via VBA after any value is entered but I don't know from where you will get a message to pop up when incorrect answer is given. Lookup table and some code maybe.

[SirName]

Well the idea behind the data validation is, when they get it wrong then it tells them what policy to read to get the correct answer. I put Select "ok" to continue that leaves the incorrect answer but I am sure some will go back and change. The main idea is for them to learn the information but I have been requested to see if I can make it unable to change. Any other suggestions? Also, can you put a hyperlink into a data validation error message.