Azazeel, as Garry has found out, there is a Windows Disc Image Burner tool built in to Windows 7. By default, it handles .iso and .img files. I have used this tool and it works fine. It's a very simple interface, but that's all that is really needed to just burn an image.
In the example you gave, you said suppose that you have an ISO image for XP and you want to make it bootable. The data in an ISO image is written to the disc in it's entirety, so there is nothing to be done to *make* it bootable. Many bootable discs are distributed as ISO files because once written to disc, they are ready to boot. If you should need to open an ISO file, you can use a program such as Virtual CloneDrive.
And, when you outgrow the burning tool in Windows 7, CDBurnerXP is a very small freeware with lots of features. At 3 MB for the installer, it is certainly not bloatware.