It depends on how you define "use those old ASP programs".
If you just want to keep running your existing ASP applications, then you should have no problem. They can even be mixed with .NET applications. That is, you can link or redirect from an ASP page to an ASPX page and vice versa.
If by "using" you mean reusing parts of your ASP pages, like includes, then this is partially possible. ASP includes can still be used within ASPX pages to some extend. However, you won't be able to call ASP functions from your ASPX pages.
Also, it's not really recommended to to do this anymore. ASP.NET now has much better ways to include content, like classes in App_Code and user controls.
If this doesn't completely answer your question, please specify "use those old ASP programs" in more detail...
Imar
---------------------------------------
Imar Spaanjaars
http://Imar.Spaanjaars.Com
Everyone is unique, except for me.
Author of
ASP.NET 2.0 Instant Results and
Beginning Dreamweaver MX / MX 2004
Want to be my colleague? Then check out this post.