As WHS pointed out,you cannot move system files as these are part of the operating system.
You can only move data files-text,photos,music,backups of your document files desktop,favorites,email messages,browser functions and the like.
These last ones require digging around in Documents and Settings and could be complicated.
However,you can move all of these using a data backup manager like GFI backup manager available HERE and burn onto a DVD,for later transfer to Win7 by copying from the DVD
As regards your current OS,you will have to lose that when installing W7,as new OS overwrite the old completely.
You could possibly dual boot both,but would be too complicated for you.
So suggest you back up your data either on to the other disk or DVD and install W7 over the current system,assuming there are no upgrade or incompatibility issues.Do a Google referring to your current OS etc.
If when installing and it turns out there are problems-STOP- otherwise you could lose your current OS without being able to install the other.
Suggest you read up on installation matters re Windows.