With Win7 Pro or Ultimate, you can download and install Virtual PC and XP mode from Microsoft and run XP for free. If your hardware supports virtualization, this might be a good bet if you can add memory to computer. Under the 64-bit OS, you need one of the following workarounds: a second computer, a dual boot computer or a virtual computer running an older version of Windows.
If the Windows version ends with 圆4, it is the 64-bit version. (This uses the 64 bit CPU) You can check by clicking Start > Control Panel > System and Security > System. Which version of Win7 do you have? (32-bit or 64-bit and Home Premium, Pro or one of the other variants) Since you indicated this under the Dell Dimension E521 with 1 GB of memory, I'll guess Win7 Pro 64-bit. Due to changes in the OS, it's hard to get the program to install in Windows 7. This is an old version of Acrobat that was written for NT, Win2K and XP.
