Presario start problem

i have a s3300nx desktop. It shut down spontaneously and started a single long beep repeating every 4-5 secs. I can hold the power button to turn it off, when I restart it does one of these in decreasing frequency...

1. all fans spool up, hard drive spins, beeps as above.
2. all fans spool up, hard drive spins, monitor comes on, then nothing
3. gets partially through startup, then either freezes or starts the beeping.
4. starts normally but crashes anywhere from 2 min to several hours later.

I have blown out all dust, unplugged all devices and peripherals, unpluged power from main cord and inside. held power button and reset just the power. this gets me to step 3 or 4 usually.

The only other info is when I came back from a trip, the cable modem was tits up. All the lights on, non responsive. I replaced it and then this started happening. Lightning strike? dunno. Thank you for your time.