This code snippet should be saved as a batch file and run in Windows. It can be set up as a scheduled task to log response times at a fixed interval. It takes one argument, the URL, which should be enclosed in quotes or Windows will barf on URLs with = (equals) signs in. If you don't supply any arguments, you will be prompted. Binary versions of curl are available via google.

If you have ISA:

REM measure response times for a site:
@echo off
IF a%1 == a (
  SET /P varHost=Enter the address, e.g. http://google.com: 
) ELSE (
  SET varHost=%1
)
SET startTime=%date% %time%
curl.exe --proxy-ntlm --proxy yourISAproxy:8080 --proxy-user username:password -s %varHost% > fulloutput.txt
echo %startTime%,%date% %time%,%varHost% >> respTimeLog.txt

If you have no proxy:

REM measure response times for a site:
@echo off
IF a%1 == a (
  SET /P varHost=Enter the address, e.g. http://google.com: 
) ELSE (
  SET varHost=%1
)
SET startTime=%date% %time%
curl -s %varHost% > fulloutput.txt
echo %startTime%,%date% %time%,%varHost% >> respTimeLog.txt