BreOl72: No clue in regard to "Area 51" (apart from the usual), but "TimeSplitters" is a console exclusive series.
If this is the 2005 FPS game and not the 1995 arcade coin op port. I think it was offered as a freebie on PC at some point.
That's how I got the game and played it many years ago.
I think this is the FPS game which somehow managed to "invert" my brains in that after this game I started playing FPS games with non-inverted (Y) controls, while earlier I had always played all my FPS games with inverted controls. It just somehow felt wrong, almost impossible, to aim with inverted controls in this particular FPS game, and after this game it felt the same to me with all FPS games, including those I had already finished a long time ago with inverted controls.
EDIT: Yeah...
https://www.mobygames.com/game/18069/area-51/
Freeware release
In October 2008 the Windows version of the game was made available free of charge. The download link can be found in the related links section.
The links in that page don't seem to work anymore so maybe it was a limited time freebie offer. I still have the installer though.