Question: I have a very large bz2 file which I want to convert to a ISO image. Are there any good open source/free programs that can do this WITHOUT having to extract it first?

**EDIT:** When I mean "without having to extract it first", I mean not having to extract the entire archive to my hard drive and then convert it to ISO.

Answer: 
> 
> When I mean "without having to extract
>  it first", I mean not having to
>  extract the entire archive to my hard
>  drive and then convert it to ISO.
> 
> 
> 

The bad news: you will have to extract it.

The good news: you don't have to extract it to your hard drive, you can do it all by piping to stdin, so it all happens in memory (which is basically what converting would do).

You'll need `mkisofs` and `bzcat` (or `bzip2 -dc`), both can be installed with [cygwin](http://www.cygwin.com/). Then it's just a matter of:

```
bzcat file.bz2 | mkisofs -o image.iso -stream-media-size #
```

where `file.bz2` is your archive and `#` is the media size in sectors. 

If your archive is actually a bzipped TAR archive, something like this would be more appropriate:

```
tar --to-stdout xjf file.tar.bz2 | mkisofs -o image.iso -stream-media-size #
```
You're using the wrong verb. You **convert** between formats with similar purposes. You could convert BZ2 into GZip; you could convert TAR.BZ2 into ZIP or RAR or 7Z. ISO is a **filesystem image**, so you need a tool that will create that for you.

In this case, you **have** a BZ2 file and you **want** an ISO containing that BZ2 file. (Or else you want an ISO containing the file that the BZ2 image contains; it's all dependent on what you want.)

ISO images happen to be the same filesystem that's burned to CDs and DVDs, so almost any CD creator software can make an ISO. (Look for the option to "Save image to drive" when you're ready to burn the files. Don't let Nero make an NRG file for you; it's not the same thing.)

In particular, I recommend:

* the `mkisofs` or `genisofs` Unix tools for command-line use or scripting;
* ImgBurn, MagicISO, or any of several other popular ISO image creators