Hi xell,
I want to make sure you understand a couple of things here. Poor hit detection in CS is not related to the feel of Windows being sluggish. The only instances where this might be the case is if you have too many programs running at once, and other similar factors that might slow down Windows. As this issue has been going on for a long time, across multiple computers and installs, it's highly unlikely there is any lasting issue that is slowing down Windows.
You've pointed out that the shots your firing are not always 100% accurate. I'm sure you're aware of this, but the guns in FPS games are not designed to be 100% accurate. They are designed to fire within a certain range of accuracy - there is an element of randomness to where the bullet will go. The further away the target, the less accurate your weapon will be - your crosshair may well be on your opponents head, but that does not mean the bullet will always hit there.
Okay - that out the way. What you're experiencing is also the result of lag/latency from a variety of factors. When you're playing an online game, this is particularly the case with FPS games, whether or not you "hit" the target depends on how good the internet connection between the you and the other player is. You may have a great internet connection, and still experience poor hit detection due to the internet connection. Any lag can be due to your own internet connection, your opponents internet connection, and the connection status of all the servers in between you and the other player. This might simply be one server, with you and the other player both directly connecting to it, or the game may involve calls to more servers - most FPS games use multiple servers for multiplayer gaming. If either your connection is bad - even if just for a fraction of a second - or your opponents connection is bad, or one of the server connections itself is bad, then you will experience latency and poor hit detection.
Whenever there is latency in multiplayer gaming, hit detection is one of the first things to suffer.
Additionally - counter strike was released by Valve in 2000; it's an old game. Unfortunately, lots of old games are not optimized to run on the latest operating systems and hardware - hardware has moved on quite a bit since 2000, and there are thousands of possible combinations of hardware. This can have an effect on game performance - your FPS might be great, but other areas of gameplay could be suffering due to newer hardware. Valve puts in a lot of work updating games, but there are too many combinations of hardware to extensively test all their games on every possible combination - this is especially the case with older games where there is less priority to update them.
All of the points above will be contributing to your issue. However, I'd also like to make it clear that this issue is limited to the game and network connections with online play. There does not appear to be any issue with your PC.
If you have tried the game on several different computers, all with the same issue, the unfortunately the game itself is the issue. Either the game not being optimized for modern hardware, lag from online play, or just the game itself not being perfect. Other than that, the issue is out of our hands - if you've got the same problem across several installations and computers, there isn't much we can do. We can rule out any issue with the computers themselves - other factors have to be considered.
Windows may feel sluggish for a variety of reasons - I suggest you monitor your RAM usage, and remove any unwanted programs from start-up. You're PC has good spec, but if you're keen to speed up Windows, you may want to consider adding a faster hard drive (7,200rpm or an SSD), or using a faster RAM type if your motherboard supports that.