One of the most commonly asked questions at the ASPMessageboard is how to upload files from the visitor's computer to the Web server. While this can be done with just ASP, it is highly recommended that a third-party component be used for any sort of heavy-duty file uploading. There are a number of components available, the best of which are listed below. You will also find articles to information on file uploading in general and how to upload files without a component!
Uploading Files Primer
Using a Third-Party Component
Advanced File Uploading (with ABCUpload)
Uploading Files without Using a Third-Party Component!
Article Collections on File Upload