Based on personal experiance I do not like HP, e-machines or gateway. If I had to chose one of the 3 though I would purchase my e-machchine again. I loved my Dell. Here is what consumer reports has to say:
Brand reliability ranked high to low:
Apple
Compaq
eMachines
Dell
HP
Sony
Gateway
Then here are their quick picks:
Best performers for gamers:
Dell Studio XPS SX8100-1986NBC $1045
Dell Inspiron I###-###-####NBC $780
Best mainstream desktops:
HP Pavilion Elite HPE-112y $800
Dell Studio XPS-7100 $700
Best budget desktops:
Dell Studio XPS 8100 $650
Compaq Presario CQ5320Y $350
Best compact desktops:
Dell Inspiron 580s $500
HP Pavilion Slimline s5310y $460
Best all-in-one desktops:
Apple iMac 27-inch MB952LL/A $1700
HP TouchSmart ###-###-#### $1150
Apple iMac 21.5-inch MB950LL/A $1200
HP TouchSmart ###-###-#### $900
For an all-in-one with a Blu-ray player:
Sony Vaio VGC-JS450F/S $1000
Best budget all-in-one:
Dell Studio One 19 (1909) $630
Hope that helps!