In Hiren's you have utilities depending of your HDD brand, seatools for seagate, maxblast for maxtor, datalifegard for WD... and usually there is a "erase", "zero", "low-level format", "wipe-out"... option. What the reset do? On a HDD data which indicate how the disk is splitted, what kind of partitons, ... and sometime you can corrupt that very low-level data, making very strange problems. As exemple, I did a very bad operation with partition magic. My HDD was detected by the BIOS, but not by windows xp setup program nor fdisk! So I had to do a low-level format and everything was fixed. But in your case I don't think it's the problem. Check your hard disk for bad sectors with testing softwares. If it find any, it's a bad hdd. Bad sectors may cause a lot of misses / retries when the HDD read so it may slow down the system.