Is my HDD failing as SMART parameter (Reallocated Sector count) is very high(390K)

Short answer: Backup your data and replace the drive as soon as possible, it is failing

Long answer: Yes, you should be worried... The drive is failing, it may die as your reading this, or it could go for months or years, but it is failing. 393k sectors is very excessive and the fact it is increasing means the drive is continuing to degrade. Acronis has a knowledge base article explaining what it is, or from Wikipedia's S.M.A.R.T. entry page:

Reallocated Sector Count
Better: lower
Severity: Critical

Count of reallocated sectors. When the hard drive finds a read/write/verification error, it marks that sector as "reallocated" and transfers data to a special reserved area (spare area). This process is also known as remapping, and reallocated sectors are called "remaps". The raw value normally represents a count of the bad sectors that have been found and remapped. Thus, the higher the attribute value, the more sectors the drive has had to reallocate. This allows a drive with bad sectors to continue operation; however, a drive which has had any reallocations at all is significantly more likely to fail in the near future.[3] While primarily used as a metric of the life expectancy of the drive, this number also affects performance. As the count of reallocated sectors increases, the read/write speed tends to become worse because the drive head is forced to seek to the reserved area whenever a remap is accessed. If sequential access speed is critical, the remapped sectors can be manually marked as bad blocks in the file system in order to prevent their use.

Seeing this parameter at a near zero value is normal, provided it does not increase over time, because that is an indication of degrading hardware and potential imminent failure. Regardless of the results of any self-testing or other information, this drive should be replaced as soon as possible to prevent catastrophic data loss.

Personally, if this value in excess of ~20 (not 20k or 200k, but 20) or an increasing value, I implement backup and hardware replacement procedures as soon as possible.

A quick check of a few drives on some computers on our test bench indicate all have their values for Reallocated Sector Count are 0, except one laptop which shows a value of 8, but it's 2 year history indicates only one reallocation event about 15 months ago.