you can try d3dwindower: //link removed//

//edit: this post has been made before the release of NewDark, and of course, the tool that has been linked is now obsolete, as the new engine can use the windowed mode natively, as mentioned in the post below and in many others.

Thankfully the new installer in 1.21 finally provides a solution, with the option to have the game force windowed when you run it with a simple edit to the cam_ext file. Just gotta open it, Ctrl F to search find "forced_windowed" and delete the ;

Bam, game can run at full huge resolution and you can still alt tab to have some youtube video playing as background noise or talk at IM, let alone having it play at something like 800x600 to do the multiple windows open at once thing. Same goes for System Shock 2, since they run on the same engine.

Wish all the games on this site were this easy to get working to comfortable multitasking perfection like this.

(I only bother leaving this clarifying post just in case there's ever anyone else out there like me who struggled with this for such an embarissingly long time and can now simply stumble upon this while google searching and find the answer.)