Dell XPS M1530 running Vista Home Premium 32-bit SP2
Intel Core 2 Duo CPU T7250 @ 2.00 ghz, 4 GB Ram, NVIDIA GeForce 8600M GT video card. Bought new in the spring/summer of 2007 (IIRC) for about $1,500. Everything about the machine is original except the battery. I bought a replacement battery from Dell sometime last year. I faithfully run CCleaner, Malwarebytes, and Microsoft Security Essentials sweeps every month and install Microsoft Security patches every week.
Last Sunday the machine would not boot. I hit the power button, heard some but not all of the usual noises, and the screen did not turn on. I found this very puzzling but assumed the problem was a dead battery despite the fact I didn't get that bootup screen that says Low Battery. I plugged it in, came back a few hours later, and everything worked fine.
I was off work all week and used the machine constantly. Had zero issues with it. For the record I turn the machine off/on a few times a day under the assumption that if I'm not using it then it shouldn't be on. Also, my machine has a very high fail rate when going into Sleep mode (I believe this is a known Vista bug), so I decided long ago that powering the machine off just makes sense.
Went to turn the machine on again today and it would not boot. Same issue. Heard some but not all of the usual noises with no response from the screen. The noises I hear are the fan and the hard drive. When the machine boots up successfully there's a crackle I assume is from the processor. I don't hear that when there's a problem. I charged the machine for a while but this didn't help. I left the machine plugged in all day and kept coming back to try it out. Many failed boot attempts ensued.
Almost 12 hours after the problem first occurred I decide to remove the battery and boot just from the power cord. This worked. Huzzah I said! It must be the battery. However, wanting to be scientific I decided I would plug the battery back in and attempt to boot up... and the machine booted just fine. Bizarre! Does my computer just NOT want to work on Sundays????????

Any suggestions?
Needless to say I'm not shutting the machine off tonight.