If Windows Update only lists that one item I'd skip SP3 since Microsoft has some method to not list it on machines that meet some criteria (sorry but I decline to list that.) As to MS07-45 the most common failure of this is some PROTECTION SOFTWARE blocked it. http://support.microsoft.com/kb/945701 is worth noting as it appears there are HOTFIXES that are not released so my advice is...
If its working, don't fix it.
On my small network, I have three machines running WinXP, SP2 version 2002, IE7, AMD powered, and according to BelArc, I have updates through 05/15/2008 from SP3, with the sole exception of MS07-045, which, inexplicably, says it needs to be reinstalled. There are no listings for SP3 in Installed Programs. I have updates set to automatic, and my question is--Do I need to install SP3 formally? Also, will the System Properties update to show that SP3 is installed?