We want to build a online compress system:
1. It has a form, in which the user can select the file to be compressed on the local computer, and then click "Start Compress" button to upload the file to our server, and our server will try compress it. There are no limitations on the size of the file to be compressed.
2. During the compress process, there should be a progress bar indicating the progress of the whole compression process.
3. We have a Windows 32bit DLL and we want to use it to performs the actual compression in the background.
4. We don't want the server performance to be impacted greatly during the compression process.
5. After the compression is completed, a link is generated so that the customer can download the compressed file.
6. Currently we have a Linux web server hosting our website. And we need to integrate the compress form in the website.
Therefore, we are seeking for a good solution for our system:
1. Which script language should we use for such a system? Php, Asp, Jsp, Perl, or other languages?
2. Should we use two servers, one for foreground website hosting, and one for background compression?
3. It seems that we must use Windows server for the background compression, as we use a Windows 32bit DLL for the actual compression, is that true?
Thank all of you.

I'm curious, what kind of "compression" are you doing? Almost every linux install can do zip/gzip/bzip out of the box.

Note, items 2 and 4 are in direct conflict.

Language is irrelevant (as long as it can load native libraries, ie your dll), you can do it with any server, pick the one your developers are most familiar with.
If you absolutely must use the windows dll, try to find out if it will work under wine on your existing linux box.

What exactly are you asking for? Someone to do it for you?

We want to get some advices so that we can have basic ideas of the project. Thereafter, we will ask someone to do the task.

What type of compression are you using? My first instinct, since you said that you have a DLL file is SharpZibLib's GZip library.