1: Yes. You can dual-boot linux and XP. 25GB should be pretty fine for normal usage. I however, don't know how to do it.
2: Yes. You can access the other partitions. They do not need to be any specific format. However, I do not believe you can make changes to the other partitions. (I have only really toyed with SLAX live CD and am always unable to make any changes to another partition except my flashdrive)
3:Since the majority of computer users use Linux/Mac, you don't have to worry about getting attacked. Linux, for the most part, is immune to such things; though not totally.
4:I don't think the windows virus' will have an effect on the linux installation. Note, they are windows virus'
5:No idea
6:I believe they do; though for what you want it for, I'd need more information.
Ubuntu is very easy for first-time Linux users
