The default PATH variable is defined in /etc/environment. To reset the path variable, open a terminal and execute the following command: . /etc/environment.
That will restore the PATH variable for the current terminal only!
To restore the .profile file copy it from the folder /etc/skel. The /etc/skel folder contains all the files that are copied to the user's home folder when a new user is created.
Also take a look at that answer: When I create a new user, how do I automatically create a .virtualenvs directory in their home directory?
If you are unable to log-in the you have to use a live-cd and then copy the content of the skeleton folder (/etc/skel) into your home directory.
Update:
To access your home or partition using a live-cd you have to mount your harddisk after you bootet up from the live-cd. Open a terminal in the live-cd and type the following:
sudo mkdir -p /mnt/root
sudo mount /dev/sda1 /mnt/root
Now your hard drive is mounted into the folder /mnt/root. Now copy the content of the skeleton folder into your home folder
cp -r /mnt/root/etc/skel/ /mnt/root/home/<your-user-name>/
The -r switch is important to copy the hidden files. Now you can unmount the disk and reboot the system cd && sync && umount /dev/sda1 && reboot (as root)
Good luck!