Are there any really good-looking PCs out there that can compete in styling with the imac?
Preferably not insanely expensive 
Try alienware. They certainly have style.
Or, preferably build your own and invest in a wicked case with see through cut outs and some glow wires.
I would agree with buying a case. There are liteally hundreds of different styles and they start fro $30 and up.
Look at Lian Li, Antec, and Coolermaster to name a few.
Or if you don’t want to build, check out a couple ads from Best Buy-type stores from the Sunday paper. Some of the PCs coming from Sony and similar are pretty snazzy.
check this beauty out
I have seen a few, some are flat screen monitors with the ‘computer’ mounted in back of the screen. I have seen a celeron the size of a cd-rom drive.
The problem with such styles is that it usually has propriatory parts, which drives up costs and limit options ( so why not just get a Mac), the exact opposite reason to get a PC.
Alienware cases are not “cool” by any stretch of the imagination, though if you’re looking for the ultimate nerd rig, then it’d be right up your alley.
The thing about an iMac, though, is there’s no tower. So if you’re looking for a pretty monitor…well I’ve never seen one. Although all the flat panel ones are bitchin’.
Didn’t Compaq/HP/someone-sold-at-best-buy make some cases not too long ago that resembled the G4? Or at least had candy-looking case components?
Shuttle make a variety of small form factor PCs. When combined with a flat panel display, they are a pretty good match up for the iMac.
Sony Vaio’s are stylish, and purple, though expensive.
Gee, Alienware and Varios are as good as Windows machines get? I am a long-time Mac user and am not impressed at all.
How hard could it be to make a stylish computer?
I am not commenting on value-for-money nor on technical issues, just design.
I had no idea there were so few (read that as none at all) good-looking Windows machines. Looks like a market niche waiting to be exploited.
Some would consider those qualities to be mutually exclusive. 
I like the Shuttle PCs which bcullman mentioned. I believe they sell barebone kits (just the case and motherboard) too. They aren’t very expandable, but neither is the iMac.
Beware of Alienware, the hardware looks good, but their service is beyond attrocious if you live outside of the USA - I tried to purchase from them but they couldn’t be bothered servicing me even though I was going “all out” on their best machine.