How to split a string based on ":" in MS-Excel?

Text to columns will work. Another option, if you want to keep the original value, is to use formulas:
in B1

=left(a1,find(":",a1)-1) 

in C1

=mid(a1,find(":",a1)+1,len(a1))

Go to Data tab, then Text to Columns option. Later, choose "Delimited" option and then select "other" and put any delimiter you want.

If you can use VBA then you can make use of the Split() function. Here's a User-Defined Function (UDF) that you can use in a cell. It splits on your choice of character and returns the nth element of the split list.

See How do I add VBA in MS Office? for information on how to define a UDF.

Function STR_SPLIT(str, sep, n) As String
    Dim V() As String
    V = Split(str, sep)
    STR_SPLIT = V(n - 1)
End Function

So you'd need to enter:

=STR_SPLIT(A1, ":", 1) // for the first half
=STR_SPLIT(A1, ":", 2) // for the second half