Freeware of the moment: CDBurnerXP Pro
Why Windows XP still doesn’t burn ISO images is a mystery to me, but I needed to just that earlier today. ISO Recorder is one of the most popular power toys to add this functionality into XP (and now for Vista with DVDs too), but I came across another option that really got the job the done well: CDBurnerXP Pro. While the word "Pro" sounds expensive, this application is completely free and is standalone, meaning it doesn’t require Roxio or any other CD-authoring software.
For my purposes, I simply chose to write an ISO image, browsed to the image on my hard drive and about four minutes later I had the end result from CDBurnerXP Pro. Oh, what did I need an ISO image for? Well, it could have been for a Vista beta, but it wasn’t….I’ll leave you guessing for a while and write it up after I actually USE the image. 
-kct
I’ve used CDBurnerXP Pro and its a great piece of software and in my opinion much more friendly than some other choices out there.
I used it today to burn an image of Ubuntu and I’m guessing you were doing something similar (just a guess though).
I’ve been using CD Burner XP Pro for some time now; excellent burner but also a great ripping and conversion tool. Couple this with Deepburner and you have most of the bases covered for free.