Global sales of personal computers tumbled by 15% year over year in the third quarter, according to a report from market intelligence firm IDC. There was a sliver of good news in that report  -- PC sales remained above their pre-pandemic levels as the industry shipped 74.3 million units in Q3, up from 70.4 million units in Q3 2019.

The bad news for any company involved in the manufacture or sale of Windows PCs is that a strong performance from Apple (AAPL 0.18%) prettied up the numbers quite a bit. Apple was the only top 5 PC provider to grow its shipments in Q3, when it shipped just over 10 million Macs, up 40% year over year.

Trouble in the land of Windows PCs

If you back Apple out of the picture, the PC market is shipping fewer units than it did before the pandemic. Unit shipments of non-Apple PCs totaled 64.2 million in Q3, down from 65.4 million in Q3 2019.

The underlying trends that drove PC sales dramatically higher during the first two years of the pandemic haven't really changed much. Many people are still working from home, and remote and hybrid work scenarios appear to be here to stay as employees continue to resist being forced back into the office.

But it's becoming clear that a big chunk of those sales earlier came from demand getting pulled forward. People who, absent the pandemic, might have waited a couple of years to upgrade their laptops found themselves using those devices every day and decided to upgrade sooner. PCs flew off the shelves. But now, there are far fewer people in such pressing need of upgrades.

The combination of pulled-forward demand and a difficult macroeconomic picture has resulted in a steep decline in PC sales. Excluding Apple, PC sales fell by 20% year over year in the third quarter.

This is bad news for HP

By 2019, the PC market had been in a slow decline for many years. The pandemic boosted the fortunes of the industry, but that boost has now faded away.

HP (HP 3.10%) is one of the biggest PC vendors. Not only did its PC sales rise during the pandemic, but profits from its PC segment ballooned as well. HP's personal systems segment generated $43.4 billion of revenue in 2021, with an adjusted operating margin of 7.2%.

That level of profitability was far higher than is typical for HP. The segment managed an adjusted operating margin of just 4.9% in 2019, and that was a significant improvement over the previous year. And remember, the PC market was stable back then. It's not right now.

I warned about the risk to HP's PC profits earlier this year. Profitability started to contract in HP's fiscal third quarter, which ended July 31. The segment's adjusted operating margin dipped to 6.9%, down 1.5 percentage points from the prior-year period. Given the historical profitability of HP's PC business, that decline was probably just the beginning.

HP stock is still trading above pre-pandemic levels, and while it looks cheap, it probably isn't. Adjusted net income reached $3.79 per share in fiscal 2021, putting its trailing price-to-earnings ratio at just 6.6. But a big chunk of the company's profits is tied to PCs, and those earnings have the potential to decline significantly. The rest of its profits come from the printing business, which likely won't fare well in a recession.

Where HP's bottom line will settle depends on the severity of the PC downturn. With non-Apple PC sales rapidly deteriorating, the rest of 2022 and 2023 likely won't be kind to HP's PC business.