Just recently a friend brought a laptop to me because it wouldn’t boot up. When I tried it the machine said there was no hard drive in it.
I was going to pull the hard drive and try it as and external drive on my machine to see it it could be salvaged as they needed some of the data if I could get it back.
When I took the panel off the bottom to remove it the hard drive had come out of the connector. I assume it had been dropped, knocked off the table or something of that sort. I plugged it back in, turned on the machine and it booted up. Since that took care of the first problem I tried a few things and the machine seemed to stop responding. I didn’t know if there was a problem from the initial disconnect of the drive or what. It was acting strange with the anti virus (Avast) so I uninstalled it and scanned for spyware and malware, there were a few so I removed those, then I did an online scan at Trend Micro which said it was clean at which point I downloaded a fresh copy of Avast and installed it.
So now the machine is clean of viruses, spyware and malware but it still kept hanging up, taking a long time to do anything so I looked at what was loading at startup and found there were a lot of things loading. Since I didn’t install the programs I had to determine what was actually needed (my rule of thumb is if it will load when I open the program I don’t need it to be running all the time) so I started disabling startup programs and the machine started working much better.
Most problems end up not being hardware. I don’t work on computers much anymore but the main thing I find when I do is spyware and malware, startup processes, a fragmented hard drive or something along those lines.
Most of the software I use for those things are linked from the anti virus page at http://www.tnt-computer.com/virus.html