Microsoft Excel is prone to a remote code-execution vulnerability. Attackers may exploit this issue by enticing victims into opening a maliciously crafted Excel file ('.xls'). Successful exploits may allow attackers to execute arbitrary code with the privileges of the user running the application. This may facilitate a compromise of vulnerable computers.
Run all software as a nonprivileged user with minimal access rights.
To reduce the impact of latent vulnerabilities, always run nonadministrative software as an unprivileged user with minimal access rights.
Do not accept or execute files from untrusted or unknown sources.
To reduce the likelihood of successful exploits, never handle files that originate from unfamiliar or untrusted sources.
Microsoft released fixes and an advisory to address this issue. Please see the references for more information.