Information about the characters that you cannot use in sites, folders, and files in SharePoint Portal Server 2003 or in SharePoint Server 2007

This article lists the characters that you cannot use in Microsoft Office SharePoint Portal Server 2003 site names, folder names, server names, and file names or in Microsoft Office SharePoint Server 2007 site names, folder names, server names, and file names.

Site names, subsite names, or site group names
You cannot use the following characters anywhere in a site name, in a subsite name, or in a site or Active Directory group name:
• tilde (~)
• number sign (#)
• percent (%)
• ampersand (&)
• asterisk (*)
• braces ({ })
• backslash (\)
• colon (:)
• angle brackets (< >)
• question mark (?)
• slash (/)
• plus sign (+)
• pipe ()
• quotation mark (")
• You cannot start a site name, subsite name, or a site group name with an underscore (_) character or with the period character.
• You cannot use the period character consecutively in the middle of a site name, a subsite name, or a site group name.
• You cannot use the period character at the end of a site name, a subsite name, or a site group name.

Folder names
You cannot use the following characters anywhere in a folder name or a server name:
• tilde
• number sign
• percent
• ampersand
• asterisk
• braces
• backslash
• colon
• angle brackets
• question mark
• slash
• pipe
• quotation mark
• You cannot use the period character consecutively in the middle of a folder name.
• You cannot use the period character at the end of a folder name.
• You cannot start a folder name with the period character.

File names
You cannot use the following characters anywhere in a file name:
• tilde
• number sign
• percent
• ampersand
• asterisk
• braces
• backslash
• colon
• angle brackets
• question mark
• slash
• pipe
• quotation mark
• You cannot use the period character consecutively in the middle of a file name.
• You cannot use the period character at the end of a file name.
• You cannot start a file name with the period character.

Source : support.microsoft.com