Hi, shankar_iyer83!
If you're using Excels versions 2007/2010 you can save the workbook as .xlsb format (binary format), that it's lighter than .xlsx or .xlsm. If you're using 2003, you should try installing first FileFormatConverters from Microsoft, it lets you use Office 2007 in advance with 2003 or previous. I used .docx and .xlsx/.xlsm but not .xlsb, even I think it works.
This is the first and easy part.
The second and harder part is analyzing what does your workbook does when it opens, if there are automatic macros executed and such things.
If first part is enough, we're done; if not, consider uploading a sample file so as to give a look at the problem.
Regards!