Runs OK if you let the computer sit overnight, but runs slow after a reboot... Sounds like something is running in the background that's hogging up the CPU time that would otherwise be going to Minecraft (like a scheduled full-system antivirus scan). When Minecraft is running slow, press CTRL+ALT+DEL and select 'Task Manager'. Use Task Manager to see what is using the most CPU and memory, it could be in 'Apps' or 'Background processes'. You probably cannot stop the process that's hogging up your CPU, but you can tell us what it is and how much CPU it's using and we might be able to help you decide whether it's a process that can be safely shut down.
By the way, it definitely isn't your computer's specs that's causing the sudden slowdown, since my son's computer is 32-bit Windows XP running on an Intel Core2 Duo with an old AGP NVidia vid card, and he gets around 40-80fps on pretty much any Minecraft version he plays.
Minecraft has a new launcher, as you would have to know to play the recent versions... However, they had a link built into the game, and the "new" launcher it linked to was beta, there is a version that is on the website, a while back I had issues, then I downloaded the new one, and my issues were resolved.
Edit: This issue stopped for a while and all was fine until a fww minutes ago. Had to update my computer again. Now it's lagging really badly again. optifine won't help because it isn't me playing the game. It's just having it open even on the title screen. I checked the background processes and nothing abnormal. I'm going to let my computer sit for a bit before I try to load minecraft again to see if that fixes it. If that doesn't work I'm going to do as suggested above and remove and reinstall the GPU driver.