Q: When I try to download a file, I get the question "What program do you want to open file with?"

A: When you need to open a package someone leaves on your doorstep, you pull out your Swiss Army knife to pick the appropriate tool; if you are at work and you have to open up a file someone sent you, your operating system gives you several options as well.

There are "no-brainer" files and some tricky ones. If you get a ".txt" file, Windows will open it with a little program called Notepad. With a ".doc" file, Word will kick in.

But it is quite possible that someone might hand you a file that is not linked with an existing program. When this happens, the Windows operating system defaults to a screen named the "open with" dialogue box. If, say, you get an ".htm" file, you can open it with your Internet Explorer browser. From then on, that file will always be "associated" with this program.

But someday you may want to open this same ".htm" file with another browser, say Netscape Communicator. To change, select the file, hold the shift key down, and select "open with." You will see a box labeled "edit file type." Click "new" to create a new file association.

Q: I am using AOL, and I can't seem to open any ".mime" or ".dat" files.

A: Simple mail transport protocol (SMTP) worked fine during the early days of the Internet. Then users wanted to send more than just text: Multipurpose Internet mail extensions (MIME) are improvements to the original way e-mail was sent on the Internet.

In 1991 this new protocol was suggested to allow audio, video, images and application programs to be sent electronically. If you are an AOL user, you can type MIME as a keyword to get tips on how to handle this file.

Files ending in ".dat" are a different story (and are not to be confused with digital audio tape). Under Windows nomenclature, ".dat" stands for data, meaning the file contains important data. We all know that Windows has a complicated database called the registry. This compendium of information on your computer is loaded from two ".dat" files: system.dat and user.dat. These files are not designed to be opened directly.

Q: Years ago I bought a Colorado tape backup Model T-1000. I sold the computer with the tape drive in it, but I still have some data on an old tape I would like to access.

A: The first thing to do is to call Las Vegas to see what the odds are on an old TR-1 magnetic tape still working!

If not stored properly, magnetic tapes have a reputation for drying out and breaking apart. But assuming it still works, you can get a new tape drive that can handle it or you can send it to a specialized service bureau to convert it to some format you can use.

The specific tape drive you have is classified as a Travan-1. The bad news is that Colorado is no longer in business; the good news is that Hewlett-Packard Co. merged with Colorado and has a tape drive for sale that can handle it--the 5-Gig tape drive (Model C4355, $219).

If you are in a situation where you don't want to buy another tape drive, you can contract out to service bureaus that can help. The granddaddy in this business is a company in Indiana called Shaftstall, 1-800-357-6250. Another source is National Data Conversion Institute, 212-463-7511. Be careful here, the minimum charges can make you think twice about the value of your old data.

John Gilroy of Item Inc. is heard on WAMU-FM radio's "The Computer Guys" at 1 p.m. on the first Tuesday of the month. Send your questions to him in care of The Washington Post, 1150 15th St. NW, Washington, D.C. 20071-5302 or via e-mail at jgilroy@iteminc.com.