It could be that software changes (additions of background programs that use lots of RAM or CPU; new drivers with performance regressions; etc) could be part of the reason. However, because this is such a vague question with so little information, it's impossible for us to guess at any specific reason why it might be so.
However, I can immediately point out in no uncertain terms that your computer is obsolete. The Celeron D is a low-end model of an obsolete processor architecture. The GeForce 6600 is a mid-range model of an obsolete GPU architecture. 2 GB of DDR1 is not only extremely limited in memory bandwidth, but the amount of RAM is insufficient for modern programs.
This assessment will be especially true if you are trying to run Windows 7 or Windows 8 with a modern browser like the latest version of Firefox, Chrome or IE. These browsers consume significantly more resources than older versions of the same, due to enabling new features and performance optimizations for computers that have more robust resources.
To put things in perspective, my smartphone (a Galaxy S5) is likely significantly faster than your desktop computer. When your computer is so old that smartphones can outperform it, it's time to upgrade.
You could certainly look into something like running an old version of Windows and an old web browser, but putting aside major security concerns related to that, this is really the wrong approach to the problem you're having. The only correct approach in my mind is to upgrade your system to a modern architecture. Even a low-end Core i3 or Celeron based on the "Haswell" microarchitecture and 4 GB of RAM, using the GPU built into the processor, would be a 10-fold increase in performance compared to what you have now. Even a $120 Chromebook would be faster!
Are you aware that the Celeron D line of processors launched in 2005? Are you aware that Intel has declared these products "End of Interactive Support", which means Intel is not working on them at all, and any issues or brokenness you encounter is entirely yours to keep?
To give you some perspective, CPUs, GPUs and RAM (and, storage subsystems like hard drives and SSDs) have evolved as much since 2005 until 2014, as the internal combustion engine has evolved from, say, 1935 to 2014. Would you be comfortable driving a 1935 car on today's roads? No? Well, why are you trying to drive a 1935 computer on the 2014 information super-highway? :)