Hi, leoslepak!
Additionally to the probable (and unpredictable until identified) virus option, it might be either Excel issues or restrictions or macros that can alter normal operation like copy & paste (or any other standard feature).
What are this issues or restrictions? Well, I was pointing towards macros that might be loaded automatically on Excel start and then affect all workbooks opened.
This macros are stored in a folder called XLSTART that is placed in Windows 7 under the path: C:UsersYourUserAppDataRoamingMicrosoftExcel (where YourUser is the user named used to identify you at PC start up... note that if you don't have to enter a password you still have a user name which you can see pressing the Start button at bottom left and it will appeared displayed at the upper right panel).
If you have another operative system version it might be stored under other path, but always named XLSTART, so you know what to search for.
When Excel starts if looks for every file under the name *.xl?? (that is to say anything that has an extension of .xls, .xla, .xlm, .xlsx, .xlsm, .xlsb, ... and I don't know if I miss any), and will load it... before opening any workbook.
Specially there is a file called PERSONAL.XLSB (or .xls? in other versions than 2007/2010) where Excel stores recorded macros to be used in all opened books.
Any of this files in XLSTART folder might have VBA code (that's to say macros, instructions, or programs let's say) that could affect or change normal Excel functioning.
That's why I've asked you to find them and upload them with a sample workbook. Why both things? Files from startup and sample workbook? Just for considering all factors inner to Excel. If we find something there you won't need to do anything else than fixing it as suggested; if we don't you'll have discarded macros malfunctioning/restrictions and can focus on virus or uninstalling and reinstalling Excel/Office programs.
Just advise if I wasn't clear enough. Don't care about asking once and once again.
Regards!
PS: in the green sticky posts you'll find the guidelines for uploading under this link
http://chandoo.org/forums/topic/posting-a-sample-workbook