I want to play MC 1.5.2, because I know how to mod it, and most of the mods I like and have are 1.5.2 mods.
Whenever I try to play, it says "Can't connect to Minecraft.com (Not downloaded)" and I realize I have to go to MC.COM and download the most recent version, but I want 1.5.2 - not 1.7.9.
It sounds like you are still using the pre-1.6 launcher, which no longer works as they dropped support for it; you'll need to download the current launcher, which as noted above allows you to play older versions without having to manually install jars.
It is also worth noting that to mod 1.5.2, which likely also applies to Forge (as back then you didn't need to do any of the following), you have to follow the steps given here to make a copy of the Minecraft jar that the launcher doesn't try to auto-update, which it will if the name matches the one of Mojang's servers (current Forge versions do this automatically when you run the installer, possibly also for late 1.5.2 versions).
Whenever I try to play, it says "Can't connect to Minecraft.com (Not downloaded)" and I realize I have to go to MC.COM and download the most recent version, but I want 1.5.2 - not 1.7.9.
What do I do?
It is also worth noting that to mod 1.5.2, which likely also applies to Forge (as back then you didn't need to do any of the following), you have to follow the steps given here to make a copy of the Minecraft jar that the launcher doesn't try to auto-update, which it will if the name matches the one of Mojang's servers (current Forge versions do this automatically when you run the installer, possibly also for late 1.5.2 versions).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?