Uninstalling unwanted software and doing a defrag of the drive will not affect your e-mails; this work can safely be done without any backing up at all.
However if you want to copy all e-mails to a safe area (flash drive or whatever) open `My Computer/C:` and go to `Tools/Folder Options/View` and select `Show hidden files and folder` click on `Apply` and `OK`.
Then `Documents and Settings/*YOU*/Local Settings/Application Data/Identities/Microsoft` and copy the whole file `Outlook Express`. If you try to edit or do anything to the file you may corrupt it and lose all your e-mails so just copy it.
If you need to copy it back at any time simply use the procedure above and copy the file to its original location.