PC shipments around the world should rise this year and next, according to the latest estimates from Gartner. But the forecast doesn't look as promising as it did a few months ago, largely due to the impact of the iPad and other tablets.

For 2010, computer shipments are expected to hit 352.4 million units, a 14.3 percent rise from 2009. But that estimate is down from Gartner's prior forecast in September in which it was eyeing growth of 17.9 percent for the year.

Continuing that trend, shipments should reach 409 million in 2011, a 15.9 percent gain from this year. But that forecast too is down from Gartner's September estimate of 18.1 percent for next year.

Gartner is pegging the lower forecasts on the iPad and other media tablets upsetting the applecart of personal computers.

"These results reflect marked reductions in expected near-term unit growth based on expectations of weaker consumer demand, due in no small part to growing user interest in media tablets such as the iPad," Ranjit Atwal, research director at Gartner, said in a statement today. "Over the longer term, media tablets are expected to displace around 10 percent of PC units by 2014."

Since its launch earlier this year, the iPad has racked up a huge number of sales. Apple sold 3 million units in the first three months alone. An analyst from Bernstein Research expects the tablet to bring in $12 billion in global sales this year followed by $20 billion next year. Some analysts have also seen or are expecting the iPad to cut into sales of portable PCs.

Beyond tablets, smartphones will also affect shipments of PCs as more people use their phones as mobile complements and even substitutes for light data use, projects Gartner. But beyond the effects of increasing demand for tablets and smartphones, a collection of forces seem to be uniting to drive down PC shipments.

With the economy still struggling to revive, consumers and businesses are putting computer purchases and replacements on hold for now. Mobile PCs for the home have seen the biggest downturn in shipments. That trend is expected to grow among mature markets and may prompt consumers to temporarily, if not permanently, turn to other mobile devices for their main computing needs, says Gartner.

As media tablets become more "PC-like" down the road, consumers may further be tempted away from buying PCs, especially Netbooks. The iPad and its brethren have already proved popular for their instant-on capability and entertainment features, according to Gartner. As tablets, phones, and other complementary devices take greater hold, the life cycle of PCs will also last longer as people feel less of a need to refresh and replace them.

Emerging markets are expected to account for 50 percent of the global PC market next year, but people in those regions might decide to bypass PCs and opt for tablets and other devices as their first computing machines.

Finally, hosted virtual desktops could start to more dramatically affect the business market by 2012, according to Gartner. That may prompt companies to rely more on refurbished PCs and thin clients to access their virtual hosts, lessening the need for new desktop PCs.