See if the Windows firewall is on.
Are you running Norton AV or Internet security?
Even if you didn't change it Norton can cause this.
Uninstall Norton and then run the cleaner.
I have 2 computers running XP Pro. I'll call mine A and my son's B. Both are connected to a router. Up until the other day we could share files and both print off one printer. Printing is ok but not sharing. Sharing suddenly stopped. I can share a folder on A and see it on B but when I try to go into the folder, something pops up telling me I might not have permission. Both A and B have one user which is an Administrator account. File and Printer Sharing is enabled on both. Both connect to the internet just fine.What could have changed? What can I look for and how can I fix this? Thanks.