Sorting out PC "sleeping" issues can be quite tricky. It is also difficult for us to help you figure it out.
To give you a little background, computer hardware has what are considered "sleep states". The preferred and most difficult to get working sleep state is S3. In this state most of the hardware powers down to either essentially off or a state where it uses very little power, including system fans, etc. The RAM is kept powered though so the system is able to quickly resume from S3 sleep.
S1 is a more "awake" state and if S3 doesn't work, S1 probably will. S1 typically requires the fans to keep running though so it isn't nearly as "nice" as S3. S1 will wake up faster than S3 though.
S4 is another useful state, known as hibernation. In this state the system does entirely shut down, except that the RAM is first copied to the hard drive and the system is configured so when you want to use it, it reloads the RAM data from the hard drive and then the system resumes. S4 uses almost no power at all and most systems do support it, even older systems. With S4 you can actually physically unplug the system and it will still be able to resume when plugged back in.
The problem with getting sleep states to work is that ALL of the hardware has to support the state and support waking from the state you are trying to use. By all hardare I mean the individual sub-components including the motherboard's LAN, motherboard's disk controllers, video card, BIOS, keyboard, USB devices, tuner cards, etc., etc.
In the BIOS you can often select if S3 is enabled or not. Sometimes you can also enable/disable S1. So, the first thing to do is make sure S3 is enabled in the BIOS. Also check with the motherboard manufacturer to see if there is a new BIOS available. It is quite common for a new BIOS to be released that fixes issues such as crashes during S3 sleep.
The next thing to do is update ALL drivers, including motherboard chipset, hard drive controllers, TV tuners, USB devices, etc. - everything.
You will often find Microsoft has hotfixes available to resolve sleep issues so search through the Microsoft knowledge bases to see if there is a hotfix that might apply to your system.
If the standard Vista settings don't work, I suggest using the MCE Standby Tool to help configure things. It can also tell you which devices support which sleep state.
A useful Vista command is powercfg. At a command prompt type powercfg -a to see what Vista "thinks" your system supports as far as sleep states. It will also sometimes tell you which hardware doesn't support S3.
Powerconfig can also provide additional information using commands such as "powercfg -devicequery s3_supported", which will tell you which devices support S3 (this command will give you an idea of how many individual "devices" are in your system that need to work properly to get S3 to work). Note that not absolutely everything needs to support S3. The system does work around or "reset" some hardware that it knows don't support S3.
When trying to figure out why a system woke up unexpectedly, "Powercfg -lastwake" can help figure that out (for when you get S3 working - partially...).
At this point you need to figure out which hardware is causing the lockups. You can disable devices in device manager or physically remove components from the computer to see if a component is causing the problem. It can be difficult to diagnose what the issues are. If the system blue screens you can sometimes get some information from analyzing the core dump files (but this is a big job). If you can figure out the culprit, sometimes the firmware can be updated if you contact the manufacturer about the issue.
Try the above suggestions and let us know if you get it working.
If all else fails, you can use either the BIOS or MCE Standby Tool to select a less useful sleep state. S1 will work with most hardware, but typically it leaves the fans running. Note that newer computers typically are configured in the BIOS to specifically support S3 and not S1 to force S3 to be used. Usually you can enable S1 in the BIOS if this is the case. S4 (hibernate) will work with all hardware, but the downside is it takes longer to resume from sleep. As long as the BIOS supports wakeup events, Media Center will configure the computer to wake up from any sleep state to record TV shows.
The next challenge is often figuring out how to make the system wake up in a friendly way. Depending on the USB hub on the motherboard and the BIOS, it is not uncommon for the Media Center remote control to fail to wake up the Media Center PC. This is annoying becuase you often have to resort to pressing the power button when you want to use the system. Note it is quite common for systems to wake properly to record, but not wake up with the remote.
What hardware are you using?
Post your hardware details and maybe someone can provide suggestions on what could be the problem. Note that most new computers do a good job at supporting S3 without any effort (it "just works"), while most older computer have trouble with S3.
STB w/R5000HD USB I/O, Gigabyte GA-P35-DS4, Quad Q6600, 4.0 GB RAM, ATI HD 3870 512MB, Ultra XVS 600W PSU, 3x SATA 500GB, 2x SATA 300GB, LG GGC-H20L, PVR-250, Toshiba 51H83 (51" HDTV), Yamaha RX-V2400 Amp, 5x Energy Speakers, SVS Sub, Harmony 880 Remote