bad blocks on hdd

You can do a Low Level Format and bad hard disk sectors blocks will be ignored. But if you have errors I would make a backup and also replace your HDD.

You can tell if your hard disk is bad with a quick S.M.A.R.T. ( Self-Monitoring, Analysis and Reporting Technolog) test with CrystalDiskInfo.
http://crystalmark.info/download/index-e.html (choose the Portable w/o Ads Simple&Update or Full)

If the number of bad blocks is too high then the drive is bad and not worth of trying to repair it. Because the bad blocks will reoccur again.

I agree with Noxho, if the drive is failing by developing bad blocks, it will continue to get worse. You already noticed that the performance is extremely slow, this is trying to tell you something that the drive is ready to fail totally. I would not use it until you back up the drive, unless there is no data that is important to you. The more you use the drive the more data you are going to lose.  You can use programs like hdd regenerator, but it is not free, this application can try to repair bad sectors if the problem is not a hardware problem. But if you get too many delay errors when running the application this means that the drive is failing. Back up your data before you lose it.

i do not agree with the above
i have repaire d many drives with HDDRegenerator, and up to now, only ONE = 1 got worse; the others never returned

It all depends on what is causing bad blocks if it is a hardware issue such as bad heads, then hdd regenerator will not repair damage to bad blocks. Hdd delays can not be fixed on a bad harddrive, it just delays the scan/repair. Delays point to pending hdd failure, especially if there are a lot of delays. If there are a lot of delays it can take several days for the HDD regenerator is finished scanning. It is cheaper just to replace the drive than taking several days to repair a drive. I too believe that HDD regenerator can save a lot of drives, I used this app lots of times myself and recovered so called bad drives, but if there are a lot of delayed sectors on the drive, the drive still will be painfully slow, if useable at all. HDD regenerator can not repair delayed sectors, it just skips over them and labels them as delayed sectors. Data saved on delayed sectors is questionable. This is why it takes so long to do things on a computer that has a number of delayed sectors.

it is obvious that physical defects cannot be repaired by ANY software - i wish they could - lol

When a disk drive starts showing bad blocks to the operating system, it is time to replace it.  The firmware in the drive makes significant efforts to re-read and error correct blocks.  When those efforts fail, the drive is going bad and it is only going to get worse.

Yes, you can revector the bad blocks using UBCD, MHDD or HDD regenerator.  However, all this does is mask the problem by mapping out the bad blocks.  The bad blocks are still there and the problem that caused them is still there.

A new drive costs from $50 to $80 if you buy an OEM "naked" drive.  It's not worth taking chances at that price.  Read everything you can off the offending drive, replace the old drive, then take a hammer to it so that you will not be tempted to use it for anything later on.

dr klahn - HDDregenerator does not "revector" sectors - it rewrites it with good amplitude

Yes, you can revector the bad blocks using UBCD, MHDD or HDD regenerator.  However, all this does is mask the problem by mapping out the bad blocks.  The bad blocks are still there and the problem that caused them is still there.

Revector-remap. Every HDD has from a very start of its production extra numbers of reserved sectors which are used to remap the bad sectors to. Usually repairing bad sectors do simply remap the bad sector read/write inquires to a sector in this reserved area.
Different programs can name this process different, tell you that it "repaired" the bad sector. But in a reality it is the operation which was supposed to be available by a HDD manufacturer. All the rest is a pure marketing.
The HDD vendor made software which makes repair bad sector task does the same.
And yes, you are right. The problem which caused the bad sectors is not resolved - the circumstances yes but the cause not.