Sounds like a problem with Notepad, but I would need to know which Operating System (OS--XP, 98, etc.) before I can give you specific instructionson how to restore Notepad. Could you please post back with that information and in the meantime we can try a couple of other things it could be. Please also do this.
1. CWS SmartKiller will interfere with HijackThis and other removal tools. Download the following file, run it and try to make a log again:CoolWWWSearch.SmartKiller removal tool
2. If that doesn't work, rename HijackThis.exe to HijackThis.com Run again and see if you can make a log.
Also, when you press Save Log, do you get a Save As dialogue box? If so make sure there is text entered into the File Name and Save As Type fields. If not, type in Hijackthis.log and/or hijackthis.txt in the File Name field. Want to make sure you're giving the file a name if HJT isn't--just something else to check.
You could also do a search of your hard drive for Hijackthis.log and hijackthis.txt and see if it's being saved somewhere that HJT doesn't know about.