Backing up System Files? Deleted Files?

Okay. So I’m mostly interested in backing up my photos and documents and such, which I’ve already done. I do have two questions though.

I’m running Windows Vista on a 6 year old laptop. I bought the protable drive for when the inevitable happens. I’m at 90% capacity on my comptuer hard drive. I’d like to delete some old files, and was wondering if I’d still have access to them when my porable drive does it’s auto back up? If I purposely delete a file and empty it from the trash can, does it still keep a copy on my portable drive, or does it delete that file, as it does its auto backup?

Also, what about system files? I realize I’ll probably have to get a new laptop when this one finally goes out on me, and I know I’ll have to get the new verion of Microsoft Office (Yes, I know about Open Office), but are there other programs/files that would be good to back up? How do I know? Or should I just start from scratch when I get a new system? How do I back up system files, and how do I knew which ones I might want/need? Am I even making any sense??

Funny! I just bought an extra h-d on the weekend. Got my docs & files all transferred over.  Do NOT want to use it as a routine back-up system … but would like to back up my computer just once – now – and then I’ll use the hd as the destination to save ALL files from here on in.  (So similar, really, to Allison’s post, which is why I’m replying to hers.  Strength in #s, right?)

How do I use the external hard-drive for backing up just once … while saving all of the files/docs that I’ve just transferred over to it?