"how would i do this?"
I'd suggest:
- Boot from the install DVD
- Run Disk Utility
- Partition the hard drive into 2 partitions. The "new" partition does not need to be large, say, 15gig or so.
- Using the install DVD, choose to install a "new" copy of the system to the NEW partition. Use the "Custom" options to install only a system, that is to say, keep it "bare bones".
- When done, create a new account there, and apply system software updates via software update.
It can come in VERY handy to have that second partition/system available. For instance, you can "switch boot" and then run Disk Utility's "repair" feature on your main partition.
To switch boot, do this:
- Restart
- As soon as you hear the startup tone, hold down the Option key and KEEP HOLDING IT DOWN
- The startup manager will appear
- Select the volume you wish to boot from and hit enter (use the pointer, the arrow keys, the tab key to move between the displayed boot volumes)
- NOTE: this doesn't change your normal boot drive (which is set in the "startup disk" preference pane
I would NOT backup to this "emergency partition". Just "let it be".
In any case, you should also have an EXTERNAL and BOOTABLE backup drive. You can create one with either CarbonCopyCloner or SuperDuper (if you haven't already done so).