procedure entry point

I have been having problems with my computer when I got it back from a friend who put more memory on it. It either freezes or reboots on me when we are on the internet.

I was downloading some updates to my computer which included Internet Explorer 8 when my computer froze. I had to hard boot it and now I get this message "The procedure entry point could not be located in the dynamic link library RPCRT4.dll." There was a lsass.exe on the top of the message (not sure if it is a capital I or lower case L). All I get after that is a black screen with my pointer visible. I tried to put my XP disk in to try and reinstall or fix it, but it won't work. Help!