Simple Screen Capturing App

We need a desktop app developing for a Windows environment. The requirement is simple, and we already have a working example which we built in-house using vbscript - it just doesn't work perfectly! The program needs to be able to load up multiple user-defined web pages at regular, user-defined intervals, and to then save a screen-capture or screen-print for each one using pre-defined naming and sequencial numbering. The interface would therefore be simple, and would need to include the ability to define multiple web page addresses to be captured (at least 10, but ideally without an upper limit, or at least with quite a large number like 25 if an upper limit is needed?). It would also need to include a scheduler - a start time and date, and an end time and date, as well as a set interval to define how regularly each image/web-page is captured. Finally, it would just need the ability to define where images were saved to. We have been running our in-house script just using screen-capture, however this obviously means that the pc running the script cannot be used for anything else whilst the program is running. If there is a way to achieve what I'm after and still be able to use the pc in question, that would be excellent, but if not, that is acceptable too. It would also be great if the program developed could be self-contained (i.e. a portable app) so that it can just be run without any install?