It's fine to ask, and I've known others to worry about it. If you boot off a flash drive (thumb drive, w/e you call it), it should not modify your hard disk unless you write to it on purpose.
If you doubt Linux's ability to correctly write to NTFS (the file system Windows uses, and this can be a concern), just don't write to it, and you might want to remount those partitions as read-only, if you mount them at all.
Edit: Error 404, post not found

Remounting partitions? What? What partitions?

Partitions contain file systems, sorry for not explaining. If you're like most people, your computer has one partition (or only one that you know of), which contains your "C:" drive. (or whatever you labeled it)
Mounting is a process which allows you to read and write files on a partition. A partition has to be mounted to be accessed. Since most OS's run from flash drives will automatically mount all partitions for reading and writing, I'm suggesting you remount it as read-only if you are afraid to write to it, and don't want anything written automatically.

Weird, the link worked for me.

I was celebrating my 404^th post

Quote from: Deep Thought on July 04, 2010, 10:38:33 am

Remounting partitions? What? What partitions?

Partitions contain file systems, sorry for not explaining. If you're like most people, your computer has one partition (or only one that you know of), which contains your "C:" drive. (or whatever you labeled it)
Mounting is a process which allows you to read and write files on a partition. A partition has to be mounted to be accessed. Since most OS's run from flash drives will automatically mount all partitions for reading and writing, I'm suggesting you remount it as read-only if you are afraid to write to it, and don't want anything written automatically.

Oh, I see.

Quote from: Deep Thought on July 04, 2010, 10:38:33 am

Weird, the link worked for me.

I was celebrating my 404^th post

Ha, didn't see that. _{222^th post!}

If you don't want to partition your hard drive and want to be able to install/uninstall your Linux OS like any other Windows program, here's a nifty little thing:

http://wubi-installer.org/

Note that it's only for Ubuntu.

Ooh, that seems useful. Thanks.