If by 'your files are backed up' you mean just data files then you need
to make an 'image' of your entire system to that same external drive.
Should your internal hard drive fail again you have the image which can
be 'restored' to a replacement internal drive. Keep the image up to date
as often as you feel you wouldn't want to loose recent files. Also the
replacement internal drive should be as large as or larger than the
original failed drive.
An image is a copy of your operating system (Windows) along with
installed applications, data, device drivers and etc. - In short
everything you need to protect against a failed internal drive.
Here is one free and well recommended offering for home use:
http://www.macrium.com/reflectfree.aspx
There are others, just Google 'free image software for windows'. There
is also a paid version that of course does a lot more but for what you
fear the freebie will do.
Don't forget to burn the CD or DVD to boot your computer in order to
restore an image, some even allow the use of a USB stick if your comp
can boot from USB. With a failed or new (but blank) internal drive you
need an auxiliary method of booting your computer to do the 'restore'.
The imaging software you choose should have ample instructions on
exactly what is needed to guard against that OOPS! kind of day ;)
HTH
John