Please provide information about how EXACTLY you are calling unwise.exe and the install.log file
It is difficult to advise you what you might be doing wrong if we don't know what you are doing.
For example, are you providing the full path to both unwise.exe AND install.log?

Wisescript.exe typeof install will create an uninstall string in registry.Some times it creates path with spaces in uninstall string.Due to this spaces in the path uninstall.exe may have problem to find the install.log file.

 You can solve this  as below;

 1.Move the install.log to any other location (say c:\test\)

2.Launch uninstall.when the exe prompt for install.log path give the new path of the install.log and proceed the uninstallation.

Is it a wise script ?

 If yes then, when you give the path of unwise.exe and INSTALL.log in script convert them to shortname (you have the option to convert into short filename in set variable action)

 set variable LOG which will have path of INSTALL.LOG and select sction to convert it to short filename
 
 If you are using some other script then use direct short filename to execute unwise.exe.

nac - this is indeed a Wisescript question, as unwise.exe is the uninstaller stub for the Wisescript EXEs.
Unwise.exe actually checks to see if the environment is 16 or 32 bit and if the latter, calls Unwise32.exe, so unless you have some really ancient operating systems to support, you can also perform the uninstall by calling Unwise32.exe direct.
Using shortfilenames, or placing double quotes around any path strings which contain spaces, is a definite "must do" - otherwise the uninstall will not find the logfile.

My point was, to check if Onkar is calling unwise.exe from any other script.