Most common reasons for this are:
1) Dirty disk
2) Security software interference
So, first off clean the game disk.
Next, disconnect from the internet, then turn off any anti-virus program and also any firewall you may have and then try the EP/SP installation.
Less common reasons for what you report are:
1) Incomplete or faulty uninstall before reinstall. (Unlikely in your case as described, as you can install base game without probs.)
2) If running Vista, trying to install via a user account other than admin.
3) If a computer with dual core processor, not setting the compatibility of already installed items. (See here:
http://modthesims2.com/wiki.php?title=Game_Help:Dual_Core_Issues for solution for that.)
