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

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

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))

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

Paste it to B1 and fill it to columns on right and rows down:

=TRIM(MID(SUBSTITUTE($A1,":",REPT(" ",999)),COLUMNS($A:A)*999-998,999))

Edit: I previously posted localized version of the formula, where ',' was replaced with ';'. That doesn't work in US-version of Excel:

=TRIM(MID(SUBSTITUTE($A1;":";REPT(" ";999));COLUMNS($A:A)*999-998;999))