If it's on a network someone might have been fiddling with the Firewall settings.
If it's on it's own perhaps windows has run an update and it altered your security settings, you might need to adjust the firewall settings to allow things to work...without having a fiddle it's hard to know.
Always worth a try: clean out your internet cache (in Internet Explorer, Choose 'Delete Browsing History' - but don't click the 'delete passwords' button) and then restart you PC.
Drew
