I have an NVidia graphics card in my Fedora 18 system and I've been using the NVidia drivers but found that the print screen and alt+print screen produced badly corrupted or all black screen shots.

Seeing as the problem has persisted through multiple upgrades of the nvidia modules on my workstation I guess that Nvidia is unwilling or unable to fix the problem under Linux.

So to remove the NVidia rpms (from rpmforge):

# change to a virtual terminal
# CTRL + ALT + F2

# login as root su - or do sudo...
su -
# remove the packages
rpm -qa | grep -i nvidia | xargs yum remove

# for some reason this failed and said 
# to continue use yum load-transaction
# the /tmp/yum*** file so I did.
# replace the X's with the actual filename
yum load-transaction /tmp/yumXXXXXXXXXXXXXXXXXXXXXXX

# reboot
shutdown -r now

# on reboot you should now be running 
# nouveau

[root@bb01 ~]# lsmod  | grep nouveau
nouveau               941736  2 
mxm_wmi                12865  1 nouveau
i2c_algo_bit           13257  1 nouveau
ttm                    79629  1 nouveau
drm_kms_helper         46708  1 nouveau
drm                   264680  4 ttm,drm_kms_helper,nouveau
i2c_core               38353  5 drm,i2c_i801,drm_kms_helper,i2c_algo_bit,nouveau
wmi                    18697  3 mxm_wmi,nouveau,asus_wmi
video                  18991  2 nouveau,asus_wmi

There may be a slight reduction in video quality / performance with the opensource driver (IMO) but the return of functional screenshots makes up for it.