Windows 7 - A Bootable USB Question...

I have a 16 gb flash drive (hoping to move up to 32 gb at some point) that I want to make bootable. I am looking for a program where I can boot into Windows XP (for virus removal situations) and then I also want to have the option to be able to boot into programs (e.g. Partition Manager, Testing tools for memory or other hardware, etc.). But at the same time I would like there to be a space where I can use it as a regular USB drive and use it to hold files. Also, do you know of a program where I can keep the files protected if I do try and plug it into a virus infected computer? And what drivers would I need to be able to make the Windows XP OS safe for any computer without running into any problems such as BSOD? This might not make much sense so if there is anything I can do to help you help me, let me know. Thanks. ^_^

This site has all info you can imagine on things bootable:
reboot.pro

Here's an example:
Windows XP + Hiren's BootCD + Windows 7 in Bootable USB Flash Drive - reboot.pro

The desire to be able to write files onto the key drive conflicts with the idea of being safe from malware. If you only boot into it, then none of the stuff on the HD of the machine should be running. It shouldn't get infected as long as you don't run any software not on the key drive. But I would advise asking questions on the site as they are the booting experts.