I have been using Tangible's Instant C# and Instant VB for several months now, and have found that both have saved me many hours' worth of manual code conversion time.  They quickly pay for themselves, and the quality of the product and the support from the company have both greatly impressed me.  I highly recommend this product to anyone who finds him or her self converting code from VB to C#, or vice versa.  This is the best tool I am aware of for source code conversion.