The two have nothing to do with one another. If your system is shutting down or rebooting without you telling it to do so, that's NOT an issue with the imposed connection limits on XP SP2.

Since you say no malware or viruses were found, I would go with it being a likely heat issue, with the chance of hardware failure. When's the last time you cracked open the case and took a good can of compressed air to it? Clearing out dust bunnies that collected on the case fan(s) and CPU fan.