If even a self-extracting exe file won't extract, you don't have a RAR problem. It feels more like something to do with data transfer, DMA or the like. Maybe if you call up MSINFO, you'll find out more.
Or: Did you mess with the BIOS lately (e. g., block transfer configuration modified or something like this)?
There's a multitude of possible explanations for this phenomenon, all of them impossible to diagnose from a distance.
Regards, Martin A
An afterthought: Could also be that you have a hardware problem (HD data cable defective/not optimally routed, a bent pin on the data cable, cable out of spec or similar). Or could be that your power supply is weak/about to die.