Please make sure it's enabled under Edit > Application Preferences > Graphics View > Context Menu.
If you cannot solve it, please attach your QCAD3.ini file if possible:
Windows XP: C:\Documents and Settings\username\Application Data\QCAD\QCAD3.ini
Windows >=7: C:\Users\username\AppData\Roaming\QCAD\QCAD3.ini

See attached:

Thanks. I wasn't able to reproduce that here though..
If you restart QCAD and right-click into an empty area of a new drawing, does the context menu show with the "Paste" item?

No nothing still.

OK. If you feel comfortable using a command line prompt (terminal), you could launch QCAD with script debugger enabled to check if there is a script exception:

qcad.exe -enable-script-debugger

Be sure to only use this for testing. The debugger should never be enabled for production use.

I've re-installed QCAD and all is working fine now, I'll monitor the situation and update you if it happens again.

Thanks.