Well, I'm not really a hardware expert, and have never claimed to be. First I have to say something is real wrong. Every program you showed me showed your +12V as 7.8-7.9. I would not believe that the computer could boot at those voltage levels. ATX standards are +/- 5%, so you are way out of acceptable range on that. Your CPU, while acceptable, the temps are a little on the high side, but that may be because of the Stock cooler. They leave a lot to be desired.
You have 8 GB of ram, but you are showing XP as a 32 bit OS, X86 (32bit) cannot address more than about 3.5-3.7 GB.
If you could, please go into BIOS and see if you have a monitoring section and tell me the readings for the +12V, +5V, and +3.3V. Also, before you buy Ram be sure to check your manual. See how much ram your board can use. I have a rig that runs that CPU and the most that board can address is 1GB per dim slot, I'm pretty sure. Also check on the specs of the ram for your board (frequency, timings and Voltage). You are always better off using a kit of ram as they are tested to work well together where individual sticks are not. Be sure to go with a Major Manufacturer too.
But, I am not sure why XP will run and windows 7 won't. While XP utilizes less resources than W7, it is still using the same hardware. When you installed Windows 7, did you have the XP drive connected?
Also, if your BIOS confirms your +12V reading, my first move would buy an new quality brand of PSU. A PSU is not the place to skimp on money. a cheap PSU will in many cases not put out what it claims to and will not have the safety features of a quality brand. And, yes, they are more expensive, but well worth it. I always recommend Seasonic, Corsair, Antec and XFX. Those are all quality brands and can be used if you decide to do a new build. You may keep that in mind when buying. Size cannot be too large, really. You will only use what your computer needs, but will always leave room for upgrades, will run cooler and last longer. BTW, of the brands I mentioned, Seasonic makes many of them and they are rebranded. When you get ready to buy, let us know and we can advise you. Plus, a Modular PSU will make the build a whole lot easier. Just a few facts to keep in mind. BTW, where you are from, I understand Be Quiet is a big name. They have just started to make an entry to the US, so I know very little about them. But, I do have a few friends in the UK and they have told me they were very good, but I have no personal experience with them nor have I heard very much about them.