I am learning new stuff myself helping you. Try this:
ECHO.| DATE >> "E:\daily backup\backuplog.txt"
xcopy "c:\documents and settings\administrator\desktop\*.rar" "E:\daily backup\" >> "E:\daily backup\backuplog.txt"
This may give you strange results though if you are prompted by XCopy, for example, I did not specify that xcopy should automatically overwrite a file, so when it prompted me with Overwrite C:\ROBOCOPYtest\test.txt (Yes/No/All)? that text was automatically written to the log file instead of displayed in the command console which can be somewhat confusing.
If you will never have to worry about overwrites, then you are good to go with these two lines. Otherwise you could append the option to XCOPY to overwrite without prompt if that's what you desire, in which case it would look like this:
ECHO.| DATE >> "E:\daily backup\backuplog.txt"
xcopy "c:\documents and settings\administrator\desktop\*.rar" "E:\daily backup\" >> "E:\daily backup\backuplog.txt" /Y
Careful that you don't run into access issues or the .bat can go into an infinite loop, in Win7 I had to run it as administrator.