Won't boot or post

So I tried a different forum and got no response. I am hoping someone from Dell can answer a question. I was using my laptop last night and it froze in the middle of a program. This has been happening fairly frequently since I updated to Windows 7 about 2 months ago. (3-4 times a week) Sometimes it fixes itself, but other times it requires a forced restart. I restarted the computer, but now it will not boot at all, no post, nothing other than the cd drive spinning up and the power light turning on. There were no beeps or flashes, the HDD light comes on for about 2 seconds and then turns off and the system hangs there indefinitely. I have tried removing the cmos battery, reseating the memory, pulling the hard drives, booting without memory and none of this changes anything, except the HDD light does not come on when there are no hard drives present (no surprise). I replaced the motherboard about one year ago through Dell, but I can't figure out what else might be the problem since I basically removed every piece of hardware I could get at without completely diassembling the computer. Does anyone have any thoughts? Must I replace the motherboard again?

As a side note, when I sent the computer off to have the motherboard replaced the first time the wifi card worked fine. Upon it's return the wifi cuts out intermittently and requires that I disable it and use an external wifi antenna which works perfectly fine. Is it possible that the motherboard they installed could have been bad or old from the beginning?