Sleep Mode Problems

I have 2 Dell pc's - a Dimension 9200 (circa 4 years old that I have just upgraded from 32 bit Vista to 64 bit Windows 7) and a new XPS8300 Windows 7 64 bit. There are connected via wired router to windows 7 home network and shared folders set up and working. The problem I have is with the 9200 machine in that when I put it into sleep mode, it seems to power up immediately after achieving sleep with hard disk activity taking place (not sure what). The green power button light is also on, rather than flashing intermittently to indicate in sleep mode.

 I have checked the power management settings and 3 devices can awaken it from sleep: keyboard, wirless and the network adaptor (ethernet). It is on balance power management (as recommended) and these are the same settings that the 8300 has. The 8300, however, does go to sleep and power buttom does flash intermittently! The only way that I have discovered to achieve sleep mode with the 9200 machine is by going to the power management tab of the network adaptor device and unchecking the "allow this device to wake computer" box. This was previously ticked, as was the sub box "only allow a magic packet to wake the computer". I have not had to uncheck this in the 8300 machine. Very confused......

Just wondered if this was ok and not going to cause any other problems and whether there was any way of overcoming my problem without having to uncheck this power management box in the 9200 machine. Apolgies is this post is in the wrong forum...