There are either of two things going on here:
Your computer can run Windows 7 but does not have enough power to run it's visual goodies, called "Aero". If you have an older or a very, very cheap computer, this is probably the case. The only way to fix this is to buy a newer computer or upgrade yours, specifically the graphics hardware.
Your friend is running a style or skin that is not native to Windows 7. She found and downloaded a style or user interface tweak that makes it act the way you're seeing. The way to fix this is to ask your friend what skin or style she is using for her Windows.
Yes, your P4 is a ten year old CPU that, while it was fast for Windows 98, and Ok for XP, is woefully underpowered when it comes to Windows 7. Dual Core doesn't tell us a lot, as there were dual core CPUs 10 years ago that would have the same problem. But it's more likely that her CPU is 5 years or younger, as that was when dual cores really become consumer level.
Also, if you're running integrated graphics from the P4 era, your graphics are completely and totally incapable of running the Windows 7 graphics. Intel probably doesn't even make drivers available for your graphics system that would work with Windows 7. I had a laptop with the Core2 chip, which followed the P4, and Intel didn't make drivers for that chipset for Windows 7, so I know they didn't do it work the P4 era graphics chips either.
You need a video card. However, because you have a P4, even with a powerful modern video card, your computer will still not really be capable of handling the Aero graphics very well. So I do NOT recommend buying a video card.
Instead, buy an inexpensive new computer with current hardware. Even a basic computer with modern hardware will be able to run Windows 7 and the Aero interface better than your current computer with a bigger video card.
