I've been a BOINC user for several yaers and been thru many versions. Never had this problem before.
When I installed 5.10.45 on my PC's (both are running XP-Pro service pack 2) the projects load normally and start-up normally. But after about a minute, BOINC suspends and all the projects stop. I've turned off all the power down functionality but there is no effect. BOINC is my screensaver. I have BOINC set to run always but it still suspends after avout a minute with no keyboard activity. It will keep running if I'm using the computer but that is not the point of having BOINC running iin the background.
I'm thinking there must be some other XP switch that I don't have set correctly. Any ideas?

Did you ever go into the preferences menu in BOINC Manager and click OK on your way out, for if you did, you're using those preferences. I don't think it has any effect on whether you tell BOINC to run always at that point.

But humor me, go into the preferences and tell if the option When computer is in use is checked or not. BOINC Manager ->Advanced view->Advanced->Preferences.

(Clicking OK saves the options, clicking Cancel doesn't, clicking Clear makes sure you have none of these options)

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BOINC stopping when use of machine stops makes me think of this option

Yes, me too. At seti@home's Computing preferences, it reads

Suspend work if no mouse/keyboard activity in last --- minutes
(Needed to enter low-power mode on some computers) 
Enforced by version 5.10.14+

only accessible via the website device profile, default 20 minutes.

So, don't forget to clear local preferences (if any).

Gruß,
Gundolf

[edit]See also this thread at seti. There the value was set to 24 hours.[/edit]

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Computer sind nicht alles im Leben. (Kleiner Scherz)

Be aware that there is a conflict with 5.10.45 and 10.5.3. I worked this out the other day with Apple. If you do not disable BOINC when you close the cover the unit will eventually not go to sleep. So when you open the cover again the screen may be blank in over 90% of the cases and you have to reboot. In some cases this can lead to "overheating" of the mac which others have experienced in unrelated posts since the unit is not really sleeping. If anyone has resolved this please let me know since Apple was unaware of this problem since they were not really familiar with BOINC.