It might help if you let us know what your system is, if you uninstalled the old anti-virus and/or anti-spyware programs, and what have you.
I've used NOD32 countless times and never had any issues, personally.
If you're having difficulty, you can hit start > run ... > msconfig > Services > Check "Hide All Microsoft Services" > Click Unselect All > Startup > Click Unselect All > OK > Restart.
This will have disabled any 3rd party software (including NOD32) and you can uninstall it from the add/remove programs link in the control panel (or Control Panel > Uninstall a Program or Programs & Features if you're in Vista)
Also, you can use system restore to restore your PC back to a previous point in time.
XP - Start computer > Keep pressing F8 > Select Safe Mode w/ Networking > Select Administrator or your user account (enter password as necessary) > Click No on the pop-up dialogue box > Select your restore point.
Vista - Start computer > Keep pressing F8 > Select Repair Your Computer > Select your user account (enter password as necessary) > Click on System Restore > Select your restore point