I totally agree with the previous posts, but in answer to your question, i think i have a solution, if this is a personal computer then i think you have a right to know what it is used for in your abscence. As the person you intend to "spy" on is your wife, then any results obtained such as credit card details should be common knowledge anyway, but i strongly advise against any immoral or illegal activity with such information.
A program i used records every keystroke and takes a screen shot at preset intervals. It automatically loads upon start-up and is invisible, only the person who installed the program knows how to bring it back up/stop it or remove it.
Do a search on www.downloads.com for "GOLDEN EYE" the publisher is ACESOFT. It is free to use for 15days then you have to buy it or remove it from your system as it is no longer functionable.
It's good to know what your kids get up to etc, but remember some things are better left unknown or as they are. Finding the facts this way is somewhat harder to deal with and can cause rift. As I found out!