Although I do agree with Kramii's answer and Joel's opinion, there are times when it is appropriate to do a rewrite. In long-lived applications (I'm talking like 10-20 years or more), maintenance becomes more and more expensive over time. This is due to the code becoming more and more spaghetti-ish as the original architecture is sacrificed for quick maintenance patches. Also, developers for older technologies become more rare and more expensive. Finally, hardware begins to age and it gets harder and harder to find new hardware, operating systems, frameworks, etc. to run the old application on top of. Also, businesses evolve, and most likely an older system will not be meeting the business needs of the organization as well as a brand new system could.

So you have to weigh all of the ongoing maintenance cost, as well as the potential benefits of a new system to the business, against the cost of rewriting the thing from scratch. Be very pessimistic in your estimates about the cost of a rewrite. It almost always costs more and takes longer than you would think.