Safe? Depends on what you mean by safe.
In Windows you have various preferences, settings, configurations, etc. all the way from your font size to your screen saver selection to your firewall settings that are all stored in the 'registry' and in various places mostly on your (C drive. If you use the restore disk to reinstall windows, you lose all that in general.
You also lose your installed programs since you can't really for the most part keep them apart from the OS install...so when you reinstall the OS, you'll then have to reinstall any stuff like drivers for her printer, any 3rd party games or utilities, etc.
If the web browser (e.g. internet explorer or firefox or whatever) has any 'saved passwords' or bookmarks or site history that is useful to you, you'll lose that when you reinstall windows unless you've carefully recorded / exported / backed it up somehow (it isn't so easy except for the bookmarks).
Unless you're very sure you've backed them up your personal files in places like "My Documents" "My pictures" etc. also go away when you reinstall windows.
Actually you typically lose everything on the drive when you reinstall windows from the restore disc, but certain files are easy enough to find and manually transfer / backup like most office documents, pictures, etc. But the preferences / configurations / browser data / installed utilities / installed drivers / installed programs are not easy to find / backup at all, and are almost inevitably lost even if you've backed up stuff like pictures / music files.
Even the music files or videos you've backed up may not play if they're associated with a DRM key that is unique to that PC, typically this may be the case for music / videos purchased online and downloaded. You may be able to export / transfer the DRM keys.
If you simply install the drive in addition to the existing one, it'll be able to be formatted and come up as a new drive letter and you can just start putting stuff there. If you move stuff from places like "My Documents" or "My Pictures" or so on various programs may not be able to find the new locations of the files anymore or quite as easily since she won't just see them in the default "My Pictures" / "My Music" / "My Documents" places a lot of the programs open / search by default.
It is possible to move the whole user profile containing "My Pictures", "My..." over to a new drive, but I don't recall an easy way for a novice to do it.
If you image copy the existing drive over to the new drive with a utility like Ghost and then expand the partition size to fill up the new drive then in theory Windows and all your programs and files will still be intact and everything will just work as it did, but you'll have more free space. This is probably the least impactful / complex way to proceed.
Here's a free beta of the latest Acronis that might help you:
http://www.acronis.com/homecom...a-testing/atiHome2009/
just click on the appropriate link on that page and it'll let you download it and give you the activation key for it.
Of course if you're willing to restore / reinstall windows, have backed up all your important files, and don't care about losing various saved data / configurations / history / installed programs then you can just reinstall windows on one drive or the other and then do whatever you want to make your backed up files accessible to the new setup....
There is a free G4L called Ghost 4 Linux, and another called clonezilla, both relating to image based backups.
there are various free backup utilities also, but mostly they are not drive / partition imaging tools:
http://www.educ.umu.se/~cobian/cobianbackup.htm
http://www.educ.umu.se/~cobian/programz/cbSetup.exe
http://www.2brightsparks.com/freeware/freeware-hub.html
http://www.2brightsparks.com/a...are/SyncBack_Setup.zip
http://www.microsoft.com/downl...92d8c52&DisplayLang=en