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Outlook 2003 introduced larger capacity storage files over previous versions. In Outlook 2000 and XP (2002) you were capped at a mere 2gb storage capacity. For people who use Outlook for all of their communications management (as I do), this was a painful issue. Under Outlook 2003 the default maximum size is now 20gb, which is significantly larger.
However, with that additional storage space comes a warning. The previous version was limited to only 2gb because of the type of data it stored: ANSI structured content. Outlook 2003 PST files can store 20gb by default, but it's an apples to oranges scenario
Outlook 2003 Support
The Bigger they are...
The larger files use a different storage structure, Unicode, which is a multi-byte sytem. This means that while it can store ten times as much in actual bytes, the underlying data capacity is closer to five times the size, since everything takes approximately twice the space internally to be stored. Some more, some less. The file is bigger, more likely to store significantly more data. And when you lose 20gb of data it's far more painful than a "mere" 2gb. Even at only 5 times that actual storage capacity, it's still exponential misery.
...The harder they fall
Back up your data completely. Back up your data often. I typically recommend backups be performed weekly for most businesses. This is not set in stone, but a weekly schedule is easier for most businesses to follow, since it can readily be added to the "Friday Routine" or equivalent. Weekly backups also avoid the horror of losing a month to six months of data in one fell swoop, should disaster strike.
Now, without further ado...
In Outlook, with the email folders or folder list viewable on the left side, right-click on the PST file (usually labeled "Personal Folders"), and select Properties.
In the properties window that opens, click the Advanced... button.
The "new" PST files are labeled simply with "Personal Folders File" as the Format.
The "older" PST files are labeled with "Personal Folders File (97-2002)" as the Format.