The only problem I ever had flashing my N900 with my Windows Vista (that's right, Vista, which got so much more flak than it deserved), is once I had it fail to use the port it had used previously for an earlier reflash. So I had to plug it in to another USB port - it installed the driver and all was good. I was a bit afraid after that that I'd literally only be able to get one use out of each usb port, but it didn't happen. From then on it almost always worked normally. The only other thing I've had to do once is skip the "start with u key pressed" stage, plug it in completely off and without u-key pressed, and then try to flash... I think. Something like that happened once, I no longer recall what caused it, but I'm pretty sure it was after I flashed a kernel image that uboot couldn't load because it was the wrong format.

Just so you know, you don't HAVE to have the u key pressed when you power it on. It is technically doable to flash without it. When the device's bootloader (Nolo) detects an in-bound flash, it handles the actual flashing, as I understand it. The u-key-pressing boot is a way to pre-set it into flashing mode just in case. It's basically a precaution on top of an already somewhat reliable process. It's a precaution that you should regularly follow, but if it's not working, you can try to flash without it, and unless something goes seriously wrong, it'll be fine.

Not sure if it would solve your device-not-found problem, though.