A standard Dell disk drive will come with as a minimum 3 partition already on it, a ESIA partition that houses the Dell diagnostics program, A recovery partition that will allow you to restore the computer to first time "out of the Box" condition and the main or OS partition. Also if Media Direct is installed that creates a 4th partition. So in reality it is not a windows 7 limitation, but rather the limits of Dells preset-up partitions. So If you want more partition space you will need to delete one or more partitions, Now the ESIA partition is very hard to get rid of and since it is so small it is usually left alone. Deleting the Recovery partition and if present the Media Direct partition would be one option or buying a larger size drive and partitioning and installing windows is another option
Recently I buyed Studio1555 with 320 Gb HD ,core 2 duo processor with windows 7 .When i contact customer care for creating new partition they told dell wit windows 7 can support totally 4 partitions of which 2 are  unaccessable partitions (Backup and Healthy) and  so u can make only 2 accessible partion(C:/,E:/). Is it right Wat they r suggesting???  If yes then Why and What sort of trouble we'll face if we intend to create more than 2 partitions..
Windows 7 will support Only 2 accesable Partitions -customer care