OP, I'd go with a Laptop instead of a bulky Desktop. I'm not sure what your budget is? So I'll recommend some laptops, at the end of this post, below the $500 mark that can run Photoshop and I'll make sure they have windows 7.
I know laptops typically have smaller screens, but you can get a 20"+ monitor, HDMI cable, and mouse to go with a Laptop and it will make using Photoshop a lot better. The largest laptop screens are about 17", which that's a good size, but a larger screen for touching up images is better IMO.
Things to look for in a laptop:
-Get a 3rd Generation Intel Processor or higher (they came out in 2012). From 3rd gen up they have Intel HD 4000+ Graphics cards built in, which are surprisingly good for video, photo editing, and even video games). An Intel i5 3xxxM series are the cheapest for a good Photoshop experience. 4th Gen 4xxx series came out around 2013 and are slightly better and improved battery life. Lastly 5th Gen is the latest 5xxx series, they just came out but are probably not yet available on lower end laptops.
-HDMI port. It's pretty standard, just make sure it has one so you can connect it to a TV or monitor.
-Memory (RAM) 4GB minimum. Look for 6GB or more for Photoshop, you'll be surprised how much memory it uses.
Since I'm recommending laptops on a budget, keep in mind they are not going to be the fastest out there. They won't have a Solid State hard-drive or the most memory. The Hard-drive and memory can be upgraded later if you need. However, You'll be stuck with the processor, which is why I stress going with an Intel i5 at minimum as apposed to an i3. AMD is another brand of processors that you'll see, but from my bias opinion are not as great as Intel so avoid them.
I found these for you:
Samsung Series 3 NP300E5C-A02US on Amazon for $413.74
Lenovo B50 Windows 7 Professional Laptop 59422966 from Amazon $500.75
Dell Inspiron i3542-6666BK from Amazon $498.00
Lenovo being my favorite. They have a good reputation for making good business class laptops and the quality and support are great too.