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Layered Service Providers (LSPs) are add-ons to the network stack in Windows that can be used to extend the behavior of the Winsock network system. LSPs are often used to perform functions such as firewalling, network content filtering and enforcing parental controls. Some malware programs are also known to install LSPs against the user's will.
Whether legitimate or not, there's always the possibility that an LSP can cause problems, either with another LSP or with other network services in the computer. The Windows firewall, for instance, might cease to function if a badly written LSP is present in the network stack. Sometimes it's not easy to determine which program has installed an offending LSP (or how to remove it), so it might be necessary to rebuild the LSP catalog manually. Remember, though, that if you do this, you will need to remove and reinstall all third-party LSPs.
To rebuild the LSP catalog in either Windows XP Service Pack 2 or Windows Server 2003 Service Pack 1 or higher, type netsh winsock reset catalog from the command line, and reboot. Before you do this, you can optionally print a list of the existing LSPs by typing netsh winsock show catalog > catalog.txt, which will write the list of LSPs to a text file named catalog.txt. This may be instrumental in determining which LSPs are giving you trouble, but there are a great many LSPs present in the network stack by default, so don't let the size of the catalog intimidate you.
If you're not using either Windows XP SP2 or Windows Server 2003 SP1, a third-party program can be used to find out more about installed LSPs. The free anti-spyware application Spybot Search & Destroy has an interactive LSP browser, and can additionally tell you which ones are valid and which are not.
Serdar Yegulalp is editor of the Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators -- and please share your thoughts as well!