How to use PowerShell select-string to find more than one pattern in a file?

I would like to search for more than one string in the files in a directory, however using "select-string -pattern" didn't help. Could anyone show me how to do it?

Example: Search all files in C:\Logs that contain the words "VendorEnquiry" and "Failed", and with a Logtime about 11:30 am. Structure of files may differ (e.g. different tag names, etc):

... <methodException>VendorEnquiry</methodException> ...
... <logTime>13/10/2010T11:30:04 am</logTime> ...
... <status>Failed</status> ...

... <serviceMethodException>VendorEnquiry</serviceMethodException> ...
... <logTime>13/10/2010</logTime> ...
... <serviceStatus>Failed</serviceStatus> ...

Thanks.