I going to presume that you're using a Wintel based PC. A number of forum members have pointed out Google desktop as a search feature plus paper tiger for document management. I guest that paper tiger comes with a strong search feature which may make google desktop redundant.
Also the next release of MS OS (Vista) is supposed to have built in search capability (similar to Apple's Spotlight). For word documents you may also wish to use the "properties" dialogue function as it provides expanded capabilities for metadata etc.
On the issue of incidental back ups you should look to the instructions or support page of the software that you are using. Most back up software has setting that allow either full or incremental back up. If not find a software package that does.