Solution:

Jeff Killed Mike

1. Jack is not the murderer, because he is the brother of the murderer.
2. Dan can't be the murderer since he ran a marathon, and the murderer recently had his leg amputated, and wouldn't be running a marathon of any magnitude that quickly.
3. Ben is not the murderer if he just met Jack, since Jack and the murderer grew up together.
4. This leaves Jeff and Mike.

Since Jeff is still alive (he wants to install a new computer next week, present tense) he must be the murderer. Mike also didn't grow up with Jack. It has been determined that Jack, Dan and Jeff are all alive. Ben must also be alive since Jeff plans to install Ben's computer next week. This means that Jeff killed Mike.

Solution:

Jason Kumar

The number on the calendar was written in a hurry.Sherlock matched the written number with the months of the year.
So the B was an 8, thereby giving us 7-8-9-10-11: July, August, September, October, November.

Use the first letter of each month and it spells J-A-S-O-N.

Solution:

2, 3, 10 and 10
To maintain the difference of eight and keeping in mind the third and fourth hint, the first and third cards can be 2 and 10 or 10 and 2. But keeping in mind the first hint, we know that the first card is 2 and the third is 10. Similarly, if we see the fifth hint, the difference between the second and fourth card should be 7. Thus we know that the second card is 3 and the fourth card is 10.

Solution:

Frost forms on the inside of the window, not the outside. So shweta could not have wiped it off to discover Mr.Ankit's body.

Solution:

The car was a convertable, and the roof was open.

Solution:

The guy was killed a long ago, but he could not realize it till he reached his room. No need to be confused. Actually the killer has somehow inserted an extremely fine and thin incision blade in the belt of the guy. The guy did not feel anything at the moment as it was extremely fine. But when he was changing his clothes and took of his pant, the incision knife that was in his body, came out stuck to the belt cutting him very finely on the insides. That is the reason there was so much blood loss that he died and it was so fine that it was not visible to the naked eyes.

Solution:

The killer is Alice.
The number in front of the month's name in the note actually points towards the position of a particular letter in every month.
Second of January = A
Third of July = L
Fourth of April = I
Second of October = C
Fourth of December = E.

Thus, the name is Alice.