First of all, have you downloaded the Arduino software
? The driver you need is in there.
Find the arduino in the device manager. If it's not in Ports(COM & LPT), look in Other Devices.
If the device doesn't say "Arduino UNO", make sure it's the right one by unplugging the Arduino - if you've found the right device, it should disappear, and then reappear when you plug it back in.
Right click and select "Update Driver"
The Hardware Update Wizard will ask if it can connect to Windows Update. Select "No, not this time" and click Next
In the next dialog, Select "Install from a specific location", and click Next
Now, select "Search for the best driver in these locations"
Check "Include this location in search"
Browse to the drivers folder inside your Arduino folder. (On my computer, it's C:\Program Files\arduino-1.0.1\drivers)
If any dialogs about Windows compatibility testing, click "Continue Anyway"
The Wizard should find the Arduino UNO driver and install it.
If the Wizard does NOT find the driver, select "Have Disk". Navigate to the \drivers\ file you found earlier, and select "Arduino UNO REV3.inf" if you have a UNO Rev 3, "Arduino UNO.inf" otherwise.
Click Finish, and see if the UNO shows up under Ports in the device manager. You may have to restart your computer.