If the Desktop is over 5 years old you may want to refer to my previous post, but some parts of this post might be useful. Also by Standard office desktop, I do not mean a gaming rig, i.e. something that likely contains one if not more high quality GPU's, that has added features up the wazoo, and potentially has certain items overclocked. I mean something you'd likely find in your office ( if your office still has desktops). Most of these depending on how recent contain CPU's that are still fairly recent, while they may not be the latest bazillion core monstrosity put out by AMD or Intel, they often preform well enough for what they are.
Now just so I am not getting anyone in trouble, if you do not own the computer yourself, i.e. if it actually is a company computer ( and you are not the owner of the company), you probably shouldn't be loading software, or adding hardware to the computer no matter how much you know what you are doing. But if you do own the computer, and if you view yourself as very very serious about crunching BOINC projects, and you are somewhat tech savvy, may I suggest picking up a decent GPU to add to your system ( making sure you stay within your systems PSU wattage amount). No I am not suggesting you go and purchase one of the $2k graphics cards which find their way into high powered computing clusters. See my previous posts about GPUs you can still get cards that are incredible work horses for under 100 dollars, while the card I got has since had several special offers on it expire, so its now closer to 100 than when I got it, it's still reasonably priced based on the price of many GPUs.
System with GPU that is able to crunch projects.
Prime Grid and Einstein - These now become a lot easier to crunch, as now Desktop computers tend to handle heat much better than laptops, and when they choose to run on the GPU the project sizes are often incredibly manageable.
GPU Grid - This is possibly my new favorite project, as it is very science and health care oriented in its goals, but as its name suggests it only runs on GPUs.
For the CPU.
I'm using the same basic write up, just trying to mark Changes
Prime Grid - If you really want to run it go ahead, this tend to cause your machine to run extra hot ( hotter than usual for boinc tasks it seems), but desktops tend to handle heat far better so it is less of an issue. Especially lately, on every machine, it tends to give bloated project sizes, with somewhat short deadlines for completion. While they do not enforce the deadlines strictly, it often causes the projects to be given High Priority status.
World Community Grid - Great choice especially if you like to support humanitarian projects. Often gives decent length time lines, and they do a good job of matching jobs to systems, with older systems usually being given less intense projects to crunch, often with shorter completion times allowing even these older system to crank out projects at a decent rate.
eOn - Not the most intriguing of projects from descriptions, but I have found on these older systems, it is my go to project. Less so now that its summer, and I brought the computer home from my office to have it crunch 24-7. I like this one for a machine that is sporadically turned on ( being on a campus with different schedules each day, and somewhat often not needing to turn on the computer for great lengths of time during the day) as especially on these older systems they seem to give out 10 minute projects. While they do not generate a lot of credits, its not a huge loss if you turn off the machine and don't turn it on for a few days and the deadline passes. But for a machine like a desktop, often left on more often, and usually being able to better handle almost all tasks, this might be overly simplistic, especially if your CPU has more than 2 cores.
Yoyo@ home - This one seems to be developing a lot of the same problems as Prime Grid.
Einstein@ home - Behaves similar to World Community Grid, but a great choice if you are fascinated with Outer Space, and the Solar System.
Climate Prediction - I almost can't believe I am recommending this project for this, because it gives incredibly huge projects. But what took me a while to realize, is the deadlines for these projects are usually a year away, and even a sparsely used system should be able to crunch 500 hours in a year. Also I need to double check this, but it seems to periodically report in and grant credits, so if you like watching the credit numbers rise, its not like you are putting off bringing in credits for a month or so.