Can somebody please tell me how to write the VBA code for this. Thank
you.

The following is a security log indicating a bunch of differents
events that occured (sample data). The actual list is much longer. I
have 4 workseets corresponding to 4 different computer logs.

Event ID Category
529 Logon/Logoff
537 Logon/Logoff
681 Account Logon
565 Directory Service Access
627 Account Management
677 Account Logon

I need to create a macro that counts how many times each Event ID
occured and put it in a new column "Instances" to look like below.
Please help.

Event ID Category Instances
529 Logon/Logoff 4
537 Logon/Logoff 78
539 Logon/Logoff 17
565 Directory Service Access 590
577 Privilege Use 1
627 Account Management 1
675 Account Logon 1852
676 Account Logon 90
677 Account Logon 121
681 Account Logon 41