Win 7 Explorer Lock ups?

You've said you tried reinstalling windows? That would be my first step to clear out the possibility of a software issue. Now, there could be many hardware problems, but most likely Its bad RAM or HD is failing. It could be the processor or mobo or video card but it's unlikely. Either bring it in to a repair store or test it yourself with something like "Hirens Ultamite boot CD" google it and burn it to a DVD. Boot from the disk and then select dos programs then testing programs then click memtest. Leave it running for about a day and If there are any errors, it's the ram. Next, run PC check from dos programs, testin tools, PC Check then select advanced tools. Check everything except the ram and only do a "smart" hd scan. If the hd smart tests show anything other than zero for the raw data for anything with th word "sector" in it, the drive is failing. As I said, it would be easier to let a coputer repair tech look at the hardware once you made sure its not the software.